Chore: update PULL_REQUEST_TEMPLATE.md for WebKit

WebKit now implements the spec

Author: marcoscaceres

.github/PULL_REQUEST_TEMPLATE.md

@@ -8,4 +8,4 @@ Implementation commitment:
 
  * [ ] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
  * [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)
- * WebKit - not implemented.
+ * [ ] WebKit (https://bugs.webkit.org/show_bug.cgi?id=)