Documentation: list git-credential in plumbing commands

Documentation: list git-credential in plumbing commands

Commit e30b2feb1b (Jun 24 2012, add 'git credential' plumbing command)
forgot to add git-credential to command-list.txt, hence the command was
not appearing in the documentation, making it hard for users to discover
it.

While we're there, capitalize the description line for git-crendential
for consistency with other commands.

Signed-off-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>