mktag doc: grammar fix, when exists -> when it exists

Amend the wording of documentation added in 6cfec03680 (mktag:
minimally update the description., 2007-06-10). It makes more sense to
say "when it exists" here, as we're referring to "the message".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-mktag.txt

@@ -29,7 +29,7 @@ has a very simple fixed format: four lines of
   tagger <tagger>
 
 followed by some 'optional' free-form message (some tags created
-by older Git may not have `tagger` line).  The message, when
+by older Git may not have `tagger` line).  The message, when it
 exists, is separated by a blank line from the header.  The
 message part may contain a signature that Git itself doesn't
 care about, but that can be verified with gpg.