mktag doc: say <hash> not <sha1>

Change the "mktag" documentation to refer to the input hash as just
"hash", not "sha1". This command has supported SHA-256 for a while
now.

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

Documentation/git-mktag.txt

@@ -23,7 +23,7 @@ Tag Format
 A tag signature file, to be fed to this command's standard input,
 has a very simple fixed format: four lines of
 
-  object <sha1>
+  object <hash>
   type <typename>
   tag <tagname>
   tagger <tagger>