Tweak the documentation of "fullscreen element ready check"

Use |element| for the variable (italic in spec) and add missing word:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=26580

NOTRY=true

Review URL: https://codereview.chromium.org/473803002

git-svn-id: svn://svn.chromium.org/blink/trunk@180249 bbb929c8-8fbe-4397-9dbb-9b2b20218538