registraion -> registration

Fixes: 0c8d3fce ("bcache: configure the asynchronous registertion to be experimental")
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Coly Li <colyli@suse.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>