refcount: Fix a kernel-doc markup

The kernel-doc markup is wrong: it is asking the tool to document
struct refcount_struct, instead of documenting typedef refcount_t.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Acked-by: Kees Cook <[e-mail]>
Link: https://lkml.kernel.org/r/afb9bb[e-mail]

include/linux/refcount.h

···
 struct mutex;
 
 /**
- * struct refcount_t - variant of atomic_t specialized for reference counts
+ * typedef refcount_t - variant of atomic_t specialized for reference counts
  * @refs: atomic_t counter field
  *
  * The counter saturates at REFCOUNT_SATURATED and will not move once