[flang][docs][NFC] Fix Markdown /*comments*/ (llvm#129018)

`*` in `/*comments*/` were interpreted as emphasis marks and were not
displayed in https://flang.llvm.org/docs/Extensions.html.

Author: kawashima-fj

flang/docs/Extensions.md

@@ -218,7 +218,7 @@ end
   the length parameter of the implicit type, not the first.
 * Outside a character literal, a comment after a continuation marker (&)
   need not begin with a comment marker (!).
-* Classic C-style /*comments*/ are skipped, so multi-language header
+* Classic C-style `/*comments*/` are skipped, so multi-language header
   files are easier to write and use.
 * $ and \ edit descriptors are supported in FORMAT to suppress newline
   output on user prompts.