blktrace: fix documentation for blk_fill_rw()

Add missing ":" after rwbs function parameter documentation that fixes
following warning :-

./kernel/trace/blktrace.c:1877: warning: Function parameter or member 'rwbs' not described in 'blk_fill_rwbs'
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 1f83bb4b ("blktrace: add blk_fill_rwbs documentation comment")
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

kernel/trace/blktrace.c

@@ -1865,7 +1865,7 @@ void blk_trace_remove_sysfs(struct device *dev)
 
 /**
  * blk_fill_rwbs - Fill the buffer rwbs by mapping op to character string.
- * @rwbs	buffer to be filled
+ * @rwbs:	buffer to be filled
  * @op:		REQ_OP_XXX for the tracepoint
  *
  * Description: