[css-syntax-3][editorial] Normalize the casing of the heading

Author: tabatkins

css-syntax-3/Overview.bs

@@ -2252,7 +2252,7 @@ Parse something according to a CSS grammar</h4>
 	</div>
 
 <h4 id="parse-comma-list">
-Parse A Comma-Separated List According To A CSS Grammar</h4>
+Parse a comma-separated list according to a CSS grammar</h4>
 
 	While one can definitely [=CSS/parse=] a value
 	according to a grammar with commas in it,