replace-version-placeholder should not recurse into the rustc-dev-guide

[pietroalbini]

Right now, running ./x run replace-version-placeholder causes among other things this unwanted diff:

diff --git a/src/doc/rustc-dev-guide/src/stabilization_guide.md b/src/doc/rustc-dev-guide/src/stabilization_guide.md
index 0ac1929..188faa8 100644
--- a/src/doc/rustc-dev-guide/src/stabilization_guide.md
+++ b/src/doc/rustc-dev-guide/src/stabilization_guide.md
@@ -109,7 +109,7 @@ to stabilize, something like (this example is taken from
 
 ```rust,ignore
 // pub(restricted) visibilities (RFC 1422)
-(active, pub_restricted, "CURRENT_RUSTC_VERSION", Some(32409)),
+(active, pub_restricted, "1.65.0", Some(32409)),
 ```
 
 The above line should be moved down to the area for "accepted"
@@ -118,13 +118,13 @@ When it is done, it should look like:
 
 ```rust,ignore
 // pub(restricted) visibilities (RFC 1422)
-(accepted, pub_restricted, "CURRENT_RUSTC_VERSION", Some(32409)),
+(accepted, pub_restricted, "1.65.0", Some(32409)),
 // note that we changed this
 ```
 
 (Even though you will encounter version numbers in the file of past changes,
 you should not put the rustc version you expect your stabilization to happen in,
-but instead `CURRENT_RUSTC_VERSION`)
+but instead `1.65.0`)
 
 ### Removing existing uses of the feature-gate

We should change it not to recurse into submodules, or at least that submodule.

cc @Mark-Simulacrum @est31

[est31]

Yeah it shouldn't recurse there. I have made it specifically avoid the list of submodules defined in tidy, but apparently rustc-dev-guide is not part of that list. I guess adding it there should be quite easy and solve the problem.

[est31]

PR filed: #102017

I didn't notice this because back when I ran the tool locally for testing, rust-lang/rustc-dev-guide#1443 hadn't been merged yet, so it wouldn't show up as change. I didn't run it again later. Anyways, I'm glad that it went so smooth so far outside of this one bug.

[pietroalbini]

Thanks for the quick fix!