git.spec.in: use README.md, not README

The file was renamed in 4ad21f5 (README: use markdown syntax,
2016-02-25), but that commit forgot to update git.spec.in, which
caused the rpmbuild target in the Makefile to fail.

Reported-by: Ron Isaacson <isaacson.ljits@gmail.com>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.spec.in

@@ -146,7 +146,7 @@ rm -rf $RPM_BUILD_ROOT
 %files -f bin-man-doc-files
 %defattr(-,root,root)
 %{_datadir}/git-core/
-%doc README COPYING Documentation/*.txt
+%doc README.md COPYING Documentation/*.txt
 %{!?_without_docs: %doc Documentation/*.html Documentation/howto}
 %{!?_without_docs: %doc Documentation/technical}
 %{_sysconfdir}/bash_completion.d