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 <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-credential.txt

@@ -3,7 +3,7 @@ git-credential(1)
 
 NAME
 ----
-git-credential - retrieve and store user credentials
+git-credential - Retrieve and store user credentials
 
 SYNOPSIS
 --------

command-list.txt

@@ -25,6 +25,7 @@ git-commit                              mainporcelain common
 git-commit-tree                         plumbingmanipulators
 git-config                              ancillarymanipulators
 git-count-objects                       ancillaryinterrogators
+git-credential                          purehelpers
 git-cvsexportcommit                     foreignscminterface
 git-cvsimport                           foreignscminterface
 git-cvsserver                           foreignscminterface