firmware: Update pointer to documentation

Documentation was revamped in 113ccc but link in
firmware_loader/main.c hasn't been updated.
Signed-off-by: NArkadiusz Drabczyk <arkadiusz@drabczyk.org>
Link: https://lore.kernel.org/r/20190912205606.31095-1-arkadiusz@drabczyk.orgSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/firmware_loader/main.c

@@ -4,7 +4,7 @@
  *
  * Copyright (c) 2003 Manuel Estrada Sainz
  *
- * Please see Documentation/firmware_class/ for more information.
+ * Please see Documentation/driver-api/firmware/ for more information.
  *
  */