Fix the histogram name, so SyncedNotification histograms show up properly.

The first attempt (https://codereview.chromium.org/26943002/) forgot the S at the end of the name SyncedNotifications.

BUG=345343

Review URL: https://codereview.chromium.org/171683016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252441 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252441