Fix section about backdating tags in the git-tag docs

The tagger is equal to the committer, not the author, so
GIT_COMMITTER_DATE is the right environment variable to use, not
GIT_AUTHOR_DATE.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-tag.txt

@@ -226,14 +226,14 @@ the tag object affects, for example, the ordering of tags in the
 gitweb interface.
 
 To set the date used in future tag objects, set the environment
-variable GIT_AUTHOR_DATE to one or more of the date and time.  The
+variable GIT_COMMITTER_DATE to one or more of the date and time.  The
 date and time can be specified in a number of ways; the most common
 is "YYYY-MM-DD HH:MM".
 
 An example follows.
 
 ------------
-$ GIT_AUTHOR_DATE="2006-10-02 10:31" git tag -s v1.0.1
+$ GIT_COMMITTER_DATE="2006-10-02 10:31" git tag -s v1.0.1
 ------------