Fix error in case-sensitivity section

Author: fantasai

selectors4/Overview.html

@@ -1462,7 +1462,7 @@ <h3 id=attribute-case><span class=secno>6.3. </span> Case-sensitivity</h3>
    depends on the document language. To match attribute values
    case-insensitively regardless of document language rules, the attribute
    selector may be immediately followed by the identifier <code>i</code>.
-   When this flag is present, UAs must match the selector's value
+   When this flag is present, UAs must match the attribute's value
    case-insensitively within the ASCII range.
 
   <div class=example>

selectors4/Overview.src.html

@@ -983,7 +983,7 @@ <h3 id=attribute-case>
   depends on the document language. To match attribute values case-insensitively
   regardless of document language rules, the attribute selector may be immediately
   followed by the identifier <code>i</code>. When this flag is present, UAs
-  must match the selector's value case-insensitively within the ASCII range.
+  must match the attribute's value case-insensitively within the ASCII range.
 
   <div class="example">
     <p>The following rule will style the <code>frame</code> attribute when it