[css-backgrounds-3] Remove redundant brackets in <bg-position>

Author: lahmatiy

css-backgrounds-3/Overview.bs

@@ -615,15 +615,15 @@ their initial position (after any <a href="#the-background-size">resizing</a>)
 within their corresponding <span>background positioning area.</span>
 
 <p>Where
-<pre class=prod><dfn><<bg-position>></dfn> = [
+<pre class=prod><dfn><<bg-position>></dfn> =
   [ left | center | right | top | bottom | <<length-percentage>> ]
 |
   [ left | center | right | <<length-percentage>> ]
   [ top | center | bottom | <<length-percentage>> ]
 |
   [ center | [ left | right ] <<length-percentage>>? ] &amp;&amp;
   [ center | [ top | bottom ] <<length-percentage>>? ]
-]</pre>
+</pre>
 
 <p>If only one value is specified, the second value is assumed to be
 ''center''. If two values are given, a <<length-percentage>> as the first