firmware: Update pointer to documentation

Documentation was revamped in 113ccc but link in
firmware_loader/main.c hasn't been updated.

Signed-off-by: Arkadiusz Drabczyk <arkadiusz@drabczyk.org>
Link: https://lore.kernel.org/r/20190912205606.31095-1-arkadiusz@drabczyk.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>