[selectors] Add a missing equals sign

Author: hediyi

selectors/Overview.bs

@@ -3097,7 +3097,7 @@ Grammar</h2>
 
 	<dfn>&lt;id-selector></dfn> = <<hash-token>>
 
-	<dfn>&lt;class-selector></dfn> '.' <<ident-token>>
+	<dfn>&lt;class-selector></dfn> = '.' <<ident-token>>
 
 	<dfn>&lt;attribute-selector></dfn> = '[' <<wq-name>> ']' |
 	                       '[' <<wq-name>> <<attr-matcher>> [ <<string-token>> | <<ident-token>> ] <<attr-modifier>>? ']'