block: fix Sphinx kernel-doc warning

Sphinx treats symbols that end with '_' as a kind of special
documentation indicator, so fix that by adding an ending '*'
to it.

../block/bio.c:404: ERROR: Unknown target name: "gfp".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/bio.c b/block/bio.c
index bf0dbe8..ae9ad34 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -400,7 +400,7 @@
 
 /**
  * bio_alloc_bioset - allocate a bio for I/O
- * @gfp_mask:   the GFP_ mask given to the slab allocator
+ * @gfp_mask:   the GFP_* mask given to the slab allocator
  * @nr_iovecs:	number of iovecs to pre-allocate
  * @bs:		the bio_set to allocate from.
  *