[css-extensions-1] Minor syntax fixes

Author: cdoublev

css-extensions-1/Overview.bs

@@ -50,8 +50,8 @@ Extension Names</h2>
 
 	<pre class='prod'>
 	<dfn>@custom-selector</dfn> = @custom-selector <<custom-selector>> <<selector-list>> ;
-	<dfn>&lt;custom-selector></dfn> = <<custom-arg>>? : <<extension-name>> [ ( <<custom-arg>>+#? ) ]? ;
-	<dfn>&lt;custom-arg></dfn> = $ <<ident-token>> ;
+	<dfn>&lt;custom-selector></dfn> = <<custom-arg>>? : <<extension-name>> [ ( <<custom-arg>>+#? ) ]?
+	<dfn>&lt;custom-arg></dfn> = '$' <<ident-token>>
 	</pre>
 
 	Where there must be no whitespace