Documentation/git-config: use bulleted list for exit codes

Documentation/git-config: use bulleted list for exit codes

Using a numbered list is confusing because the exit codes are not listed
in order so the numbers at the start of each line do not match the exit
codes described by the following text.  Switch to a bulleted list so
that the only number appearing on each line is the exit code described.

Signed-off-by: John Keeping <redacted>
Signed-off-by: Junio C Hamano <redacted>