io_uring/mock: Fix typo in help text

Fix the spelling of "subsystem".

Signed-off-by: J. Neuschäfer <j.ne@posteo.net>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: neuschaefer

init/Kconfig

@@ -1897,7 +1897,7 @@ config IO_URING_MOCK_FILE
 	default n
 	depends on IO_URING
 	help
-	  Enable mock files for io_uring subststem testing. The ABI might
+	  Enable mock files for io_uring subsystem testing. The ABI might
 	  still change, so it's still experimental and should only be enabled
 	  for specific test purposes.