[selectors] Forgot about the attribute-selector modifier.

Author: tabatkins

selectors/Overview.bs

@@ -3102,11 +3102,13 @@ Grammar</h2>
 	<dfn>&lt;class-selector></dfn> '.' <<ident-token>>
 
 	<dfn>&lt;attribute-selector></dfn> = '[' <<wq-name>> ']' |
-	                       '[' <<wq-name>> <<attr-matcher>> [ <<string-token>> | <<ident-token>> ] ']'
+	                       '[' <<wq-name>> <<attr-matcher>> [ <<string-token>> | <<ident-token>> ] <<attr-modifier>>? ']'
 
 	<dfn>&lt;attr-matcher></dfn> = '=' | <<include-match-token>> | <<dash-match-token>> | <<prefix-match-token>> |
 	                 <<suffix-match-token>> | <<substring-match-token>>
 
+	<dfn>&lt;attr-modifier></dfn> = i
+
 	<dfn>&lt;pseudo-class-selector></dfn> = ':' <<ident-token>> |
 	                          ':' <<function-token>> <<any-value>> ')'