docs: fix yaml refman example of embedding links in a code comment

With the previous commit, we made this smartly detect when parentheses
are not needed. But the example was broken, because it doesn't follow
its own documented rules to use `[[#` syntax. Add the missing hash
character.

docs/markdown/Yaml-RefMan.md

@@ -48,7 +48,7 @@ Examples:
 Now the same in a code block:
 
 ```meson
-[[#@str]] [[executable]]('main', [
+[[#@str]] [[#executable]]('main', [
     'file_@0@.cpp'.format([[#meson.version]])
 ])
 ```