Update "describe" documentation to match reality

A sample "git describe -h" did not match what the program actually says.

Signed-off-by: Thiago Farina <tfransosi@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitcli.txt

@@ -81,7 +81,7 @@ couple of magic command line options:
 +
 ---------------------------------------------
 $ git describe -h
-usage: git-describe [options] <committish>*
+usage: git describe [options] <committish>*
 
     --contains            find the tag that comes after the commit
     --debug               debug search strategy on stderr