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

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: Junio C Hamano <gitster@pobox.com>