Fixed update-hook example allow-users format.

Fixed update-hook example allow-users format.

The example provided with the update-hook-example does not work on
either bash 2.05b.0(1)-release nor 3.1.17(1)-release. The matcher did
not match the lines that it advertised to match, such as:

refs/heads/bw/        linus
refs/heads/tmp/*      *

In POSIX 1003.2 regular expressions, the star (*), is not an wildcard
meaning "match everything", it matches 0 or more matches of the atom
preceding it.

So to match "refs/heads/bw/topic-branch", the matcher should be written
as "refs/heads/bw/.*" to match "refs/heads/bw/" and everything after it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>