http-backend: more explict LocationMatch

In the git-http-backend examples, only match git-receive-pack within
/git/.

Signed-off-by: Mark Lodato <lodatom@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-http-backend.txt

@@ -69,7 +69,7 @@ To enable anonymous read access but authenticated write access,
 require authorization with a LocationMatch directive:
 +
 ----------------------------------------------------------------
-<LocationMatch ".*/git-receive-pack$">
+<LocationMatch "^/git/.*/git-receive-pack$">
 	AuthType Basic
 	AuthName "Git Access"
 	Require group committers