drivers/block/Kconfig: update RAM block device module name

RAM block device support module name changed to brd.ko some years ago
with an "rd" alias to match previous module implementation. This patch
updates its Kconfig definition.

Signed-off-by: Fabian Frederick <[e-mail]>
Acked-by: Randy Dunlap <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/block/Kconfig

···
 	  For details, read <file:Documentation/blockdev/ramdisk.txt>.
 
 	  To compile this driver as a module, choose M here: the
-	  module will be called rd.
+	  module will be called brd. An alias "rd" has been defined
+	  for historical reasons.
 
 	  Most normal users won't need the RAM disk functionality, and can
 	  thus say N here.