git-for-each-ref.txt: 'raw' is a supported date format

Commit 7dff9b3 (Support 'raw' date format) added a raw date format.
Update the git-for-each-ref documentation to include this.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-for-each-ref.txt

@@ -117,7 +117,7 @@ returns an empty string instead.
 
 As a special case for the date-type fields, you may specify a format for
 the date by adding one of `:default`, `:relative`, `:short`, `:local`,
-`:iso8601` or `:rfc2822` to the end of the fieldname; e.g.
+`:iso8601`, `:rfc2822` or `:raw` to the end of the fieldname; e.g.
 `%(taggerdate:relative)`.