Change docs for DeprecatedGetPrefRegistry to point to the correct approach.

BUG=324520

Review URL: https://codereview.chromium.org/99123002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238109 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238109