Fix documentation grammar typo

Introduced in 492cf3f (More precise description of 'git describe --abbrev', 2009-10-29)

Signed-off-by: Gisle Aas <gisle@aas.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-describe.txt

@@ -120,7 +120,7 @@ closest tagname without any suffix:
 	tags/v1.0.0
 
 Note that the suffix you get if you type these commands today may be
-longer than what Linus saw above when he ran this command, as your
+longer than what Linus saw above when he ran these commands, as your
 git repository may have new commits whose object names begin with
 975b that did not exist back then, and "-g975b" suffix alone may not
 be sufficient to disambiguate these commits.