README.md: don't call git stupid in the title

"the stupid content tracker" was true in the early days of Git, but
hardly applicable these days. "fast, scalable, distributed" describes
Git more accuralety.

Also, "stupid" can be seen as offensive by some people. Let's not use it
in the very first words of the README.

The new formulation is taken from the description of the Debian package.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

README.md

-Git - the stupid content tracker
-================================
+Git - fast, scalable, distributed revision control system
+=========================================================
 "git" can mean anything, depending on your mood.