Say "reserved" rather than "reserving" in title

Since all other chapter titles are either noun
phrases (e.g. "additions to...", "X change", etc.) or verb phrases in
imperative mood (e.g. "remove...", "reject...", "combine..."), it
would be odd to have a stray chapter title in the present continuous
tense ("reserving...").  Staying with a noun sounds better to my ear
than an imperative verb phrase here, so let's do that.

This "reserving..." chapter title was also used in the Rust 2021
guide.  For consistency, let's change that title and URL also, and add
a redirect.

book.toml

@@ -80,6 +80,7 @@ search.use-boolean-and = true
 "/rust-2018/platform-and-target-support/msvc-toolchain-support.html" = "../../../rustc/platform-support.html"
 "/rust-2018/platform-and-target-support/musl-support-for-fully-static-binaries.html" = "../../../rustc/platform-support.html"
 "/rust-2018/platform-and-target-support/cdylib-crates-for-c-interoperability.html" = "https://github.com/rust-lang/rfcs/blob/master/text/1510-cdylib.md"
+"/rust-2021/reserving-syntax.html" = "reserved-syntax.html"
 "/rust-next/index.html" = "../rust-2021/index.html"
 "/rust-next/edition-changes.html" = "../rust-2021/index.html"
 "/rust-next/dbg-macro.html" = "../../std/macro.dbg.html"

src/SUMMARY.md

@@ -30,7 +30,7 @@
   - [IntoIterator for arrays](rust-2021/IntoIterator-for-arrays.md)
   - [Disjoint capture in closures](rust-2021/disjoint-capture-in-closures.md)
   - [Panic macro consistency](rust-2021/panic-macro-consistency.md)
-  - [Reserving syntax](rust-2021/reserving-syntax.md)
+  - [Reserved syntax](rust-2021/reserved-syntax.md)
   - [Warnings promoted to errors](rust-2021/warnings-promoted-to-error.md)
   - [Or patterns in macro-rules](rust-2021/or-patterns-macro-rules.md)
   - [C-string literals](rust-2021/c-string-literals.md)
@@ -55,4 +55,4 @@
   - [Unsafe `extern` blocks](rust-2024/unsafe-extern.md)
   - [Unsafe attributes](rust-2024/unsafe-attributes.md)
   - [Rustdoc combined tests](rust-2024/rustdoc-doctests.md)
-  - [Reserving syntax](rust-2024/reserving-syntax.md)
+  - [Reserved syntax](rust-2024/reserved-syntax.md)

src/rust-2021/c-string-literals.md

@@ -69,4 +69,4 @@ Migration is only necessary for macros which may have been assuming a sequence o
 As part of the [syntax reservation] for the 2021 edition, any macro input which may run into this issue should issue a warning from the `rust_2021_prefixes_incompatible_syntax` migration lint.
 See that chapter for more detail.
 
-[syntax reservation]: reserving-syntax.md
+[syntax reservation]: reserved-syntax.md

src/rust-2021/reserving-syntax.md → src/rust-2021/reserved-syntax.md

@@ -1,4 +1,4 @@
-# Reserving syntax
+# Reserved syntax
 
 ## Summary
 

src/rust-2024/reserving-syntax.md → src/rust-2024/reserved-syntax.md

@@ -1,4 +1,4 @@
-# Reserving syntax
+# Reserved syntax
 
 🚧 The 2024 Edition has not yet been released and hence this section is still "under construction".
 More information may be found in the tracking issue at <https://github.com/rust-lang/rust/issues/123735>.
@@ -39,7 +39,7 @@ three tokens
 
 Starting in the 2024 Edition, the `#"foo"#` line and the `###` line now generates a compile error because those forms are now reserved.
 
-[2021]: ../rust-2021/reserving-syntax.md
+[2021]: ../rust-2021/reserved-syntax.md
 [string literal]: ../../reference/tokens.html#string-literals
 [RFC 3593]: https://rust-lang.github.io/rfcs/3593-unprefixed-guarded-strings.html