[css-paint-api] Changes |constructor| variable to |paintCtor| in registerPaint algorithm.

Fixes #328.

Author: bfgeek

css-paint-api/Overview.bs

@@ -194,7 +194,7 @@ called, the user agent <em>must</em> run the following steps:
 
         - <a>paint image name</a> being |name|
 
-        - <a>class constructor</a> being |constructor|
+        - <a>class constructor</a> being |paintCtor|
 
         - <a>paint function</a> being |paint|