Update core/dom/README.md about Fullscreen

Bug: 738794
Change-Id: I6926dd2db27f0bd824e4b4566278af27c9feb5cb
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/563304Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485197}

third_party/WebKit/Source/core/dom/README.md

@@ -136,7 +136,7 @@ appropriate directory, including, but not limited to:
 - PseudoElementData
 - VisitedLinkState (HTMLLinkElement)
 
-## fullscreen (-> core/fullscreen or core/html)
+## [Fullscreen](https://fullscreen.spec.whatwg.org/) (-> core/fullscreen)
 
 - DocumentFullscreen
 - ElementFullscreen