[clang-tidy][docs] Replace _not_ in reserved-identifier.rst with *not* (#112162)

Fixes a documentation formatting error where `_not_` was used which has no
special meaning in reST and replaces it with `*not*`.

Closes #111691.

Author: njriasan

clang-tools-extra/docs/clang-tidy/checks/bugprone/reserved-identifier.rst

@@ -28,7 +28,7 @@ Violating the naming rules above results in undefined behavior.
   int _g(); // disallowed in global namespace only
 
 The check can also be inverted, i.e. it can be configured to flag any
-identifier that is _not_ a reserved identifier. This mode is for use by e.g.
+identifier that is *not* a reserved identifier. This mode is for use by e.g.
 standard library implementors, to ensure they don't infringe on the user
 namespace.