HACKING: Fix instruction for git pull --rebase

HACKING: Fix instruction for git pull --rebase

When following the Patch Guidelines step by step,
an error occurs at step 6.

"git pull --rebase origin/master" results in the error:

fatal: 'origin/master' does not appear to be a git repository

Removing the / seems to fix this.

Change-Id: I4e2fa23c60654abeaebd3b25a8c8375aa07b0abd
Signed-off-by: Bill Traynor <wmat@alphatroop.com>
Signed-off-by: Peter Stuge <peter@stuge.se>
Reviewed-on: http://openocd.zylin.com/574
Reviewed-by: David Anders <danders.dev@gmail.com>
Tested-by: jenkins