Capitalize first letter of a sentence (s/if/If/)

Example 2 has a sentence that starts out "if we call" (lowercase).  This commit fixes that to start with a capital "I" in "If".

Author: dholbert

css-properties-values-api/Overview.bs

@@ -287,7 +287,7 @@ This can change the set of <a>declared values</a> which requires the <a>cascade<
 	of the property, which in the case of color is ''inherit''. Although there was a valid usable value
 	(green), this was removed during parsing because it was superseded by the URL.
 
-	if we call:
+	If we call:
 
 	<pre class='lang-javascript'>
 	CSS.registerProperty({