shell/Readme.md: Replace `which` with `command -v`

I just learned (via the Git list [1]) that `which` is not part of POSIX.  The
POSIX 2008 equivalent is `command -v` [2].  We can make the shell introduction
more portable by sticking to the POSIX utilities [3].

I can't think of a system that *hasn't* had a `which` [4,5], but there's bound
to be one sooner or later ;).

[1]: http://article.gmane.org/gmane.comp.version-control.git/216007
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html
[3]:
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap04.html#tag_20
[4]: Gentoo uses the GNU implementation developed by Carlo Wood [6]:
    This implementation seems to have used a which 1.0 by Paul Vixie
    as a starting point [7] in 1999, but I've been unable to track
    down any history for Paul's version.
[5]: Even my university's archaic SunOS 5.10 server has a csh
    implementation of `which`, (c) 1997, by Sun Microsystems, Inc.
[6]: http://savannah.gnu.org/projects/which
    http://carlo17.home.xs4all.nl/which/
[7]:
http://cvs.savannah.gnu.org/viewvc/which/README.in?revision=1.1&root=which&view=markup