Fix notifications typo in chrome://flags.

BUG=340371
NOTRY=true

Review URL: https://codereview.chromium.org/149053011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248743 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248743