[css-values-5][editorial] fix caps

Author: tabatkins

css-values-5/Overview.bs

@@ -1071,7 +1071,7 @@ Argument Ranges</h4>
 	the result is NaN.
 
 <h3 id=random-item>
-Picking A Random Item From A List: the ''random-item()'' function</h3>
+Picking a Random Item From a List: the ''random-item()'' function</h3>
 
 	The <dfn>random-item()</dfn> function
 	resolves to a random item