patch 9.0.0659: wrong type of comment in SetSyn() function

Problem:    Wrong type of comment in SetSyn() function.
Solution:   Use Vim9 comment. (closes #11278)
diff --git a/runtime/synmenu.vim b/runtime/synmenu.vim
index dd191d8..947b16c 100644
--- a/runtime/synmenu.vim
+++ b/runtime/synmenu.vim
@@ -14,7 +14,7 @@
     filetype = "fvwm"
   endif
   if name == "whitespace"
-    " do not replace the filetype but add whitespace on top
+    # do not replace the filetype but add whitespace on top
     filetype = &ft .. ".whitespace"
   endif
   if !exists("s:syntax_menu_synonly")