Documentation: complicate example of "man git-command"

Documentation: complicate example of "man git-command"

The manual page for the command invoked as "git clone" is named
git-clone(1), and similarly for the rest of the git commands.
Make sure our first example of this in tutorials makes it clear
that it is the first two words of a command line that make up the
command's name (that is: for example, the effect of "git svn
dcommit" is described in git-svn(1)).

Signed-off-by: Jonathan Nieder <jrnieder@uchicago.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>