Documentation/config: fix replacement for --get-urlmatch

Documentation/config: fix replacement for --get-urlmatch

The documentation claims that --get-urlmatch is replaced by

  git config get --all --show-names --url=<URL> <name>

However, --url cannot be combined with --all, and this command
fails in practice.

Update the replacement to use only --url, which matches the
actual behavior of --get-urlmatch.

Signed-off-by: Pushkar Singh <pushkarkumarsingh1970@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>