git-rm.txt: Fix quoting

git-rm.txt: Fix quoting

Literal " produces typographically incorrect quotations, but "works" in
most circumstances. In the subheadings of git-rm.txt, it "works" for the
html backend but not for the docbook conversion to nroff: double "" and
spurious double spaces appear in the output.

Replace "incorrect" quotations by ``correct'' ones, and fix other
"quotations" which are really `code fragments`.

This should make git-rm.txt "-clean.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>