Bug 1729011 - Change dead link to MDN in nsCOMPtr.h to reference source docs. r=xpcom-reviewers,mccr8 DONTBUILD

Differential Revision: https://phabricator.services.mozilla.com/D124461

xpcom/base/nsCOMPtr.h

@@ -10,8 +10,8 @@
 /*
  * Having problems?
  *
- * See the User Manual at:
- *   https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Using_nsCOMPtr
+ * See the documentation at:
+ *   https://firefox-source-docs.mozilla.org/xpcom/refptr.html
  *
  *
  * nsCOMPtr