[css2][editorial] change a ' to ' so it doesn't mess up Bikeshed syntax highlighting for the entire doc

Author: tabatkins

css2/Overview.bs

@@ -2830,7 +2830,7 @@ inside double quotes, unless escaped (e.g., as ''\"'' or as
 "this is a 'string'"
 "this is a \"string\""
 'this is a "string"'
-'this is a \'string\''
+'this is a \'string\''
 </pre>
 </div>