Update PULL_REQUEST_TEMPLATE.md

Author: marcoscaceres

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,7 +5,7 @@ The following tasks have been completed:
  * [ ] Modified Web platform tests (link to pull request)
 
 Implementation commitment:
-
- * [ ] WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
+ 
  * [ ] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
  * [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)
+ * WebKit - not implemented.