Fix typo in bandwidth log command line option

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/init.c b/init.c
index 4d0a906..40a17ac 100644
--- a/init.c
+++ b/init.c
@@ -1076,7 +1076,7 @@
 		case 'l':
 			write_lat_log = 1;
 			break;
-		case 'w':
+		case 'b':
 			write_bw_log = 1;
 			break;
 		case 'o':