semaphore: fix improper comment reference to mutex

Replace "mutex" with "semaphore" in down_trylock comment

Signed-off-by: Lucia Rosculete <luciarosculete@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

kernel/semaphore.c

@@ -118,7 +118,7 @@ EXPORT_SYMBOL(down_killable);
  * down_trylock - try to acquire the semaphore, without waiting
  * @sem: the semaphore to be acquired
  *
- * Try to acquire the semaphore atomically.  Returns 0 if the mutex has
+ * Try to acquire the semaphore atomically.  Returns 0 if the semaphore has
  * been acquired successfully or 1 if it it cannot be acquired.
  *
  * NOTE: This return value is inverted from both spin_trylock and