Documentation/git-config.txt: AsciiDoc tweak to avoid leading dot

Bram Schoenmakers noticed that git-config document was formatted
incorrectly.  Depending on the version of AsciiDoc and docbook
toolchain, it is sometimes taken as a numbered example by AsciiDoc,
some other times passed intact to roff format to confuse "man".

Since we refer to the repository metadata directory as $GIT_DIR
elsewhere, work it around by using that symbolic name.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-config.txt

@@ -142,7 +142,7 @@ FILES
 If not set explicitly with '--file', there are three files where
 git-config will search for configuration options:
 
-.git/config::
+$GIT_DIR/config::
 	Repository specific configuration file. (The filename is
 	of course relative to the repository root, not the working
 	directory.)