refactor(styles): make styles example use a const

This makes it easier to copy this example for use in the derive API,
like so:

```rust
const STYLES: Styles = Styles::styled()
    .header(AnsiColor::Green.on_default().bold())
    .usage(AnsiColor::Green.on_default().bold())
    .literal(AnsiColor::Blue.on_default().bold())
    .placeholder(AnsiColor::Cyan.on_default());

#[derive(Parser)]
#[clap(styles = STYLES)]
struct Cmd {
  ...
}
```

If you use the `|` method then it's not a constant function.

clap_builder/src/builder/command.rs

@@ -1199,13 +1199,13 @@ impl Command {
     /// ```no_run
     /// # use clap_builder as clap;
     /// # use clap::{Command, ColorChoice, builder::styling};
-    /// let styles = styling::Styles::styled()
-    ///     .header(styling::AnsiColor::Green.on_default() | styling::Effects::BOLD)
-    ///     .usage(styling::AnsiColor::Green.on_default() | styling::Effects::BOLD)
-    ///     .literal(styling::AnsiColor::Blue.on_default() | styling::Effects::BOLD)
+    /// const STYLES: styling::Styles = styling::Styles::styled()
+    ///     .header(styling::AnsiColor::Green.on_default().bold())
+    ///     .usage(styling::AnsiColor::Green.on_default().bold())
+    ///     .literal(styling::AnsiColor::Blue.on_default().bold())
     ///     .placeholder(styling::AnsiColor::Cyan.on_default());
     /// Command::new("myprog")
-    ///     .styles(styles)
+    ///     .styles(STYLES)
     ///     .get_matches();
     /// ```
     #[cfg(feature = "color")]

src/bin/stdio-fixture.rs

@@ -15,12 +15,12 @@ fn main() {
     #[cfg(feature = "color")]
     {
         use clap::builder::styling;
-        let styles = styling::Styles::styled()
-            .header(styling::AnsiColor::Green.on_default() | styling::Effects::BOLD)
-            .usage(styling::AnsiColor::Green.on_default() | styling::Effects::BOLD)
-            .literal(styling::AnsiColor::Blue.on_default() | styling::Effects::BOLD)
+        const STYLES: styling::Styles = styling::Styles::styled()
+            .header(styling::AnsiColor::Green.on_default().bold())
+            .usage(styling::AnsiColor::Green.on_default().bold())
+            .literal(styling::AnsiColor::Blue.on_default().bold())
             .placeholder(styling::AnsiColor::Cyan.on_default());
-        cmd = cmd.styles(styles);
+        cmd = cmd.styles(STYLES);
     }
     cmd.get_matches();
 }