sched: Fix docbook parameter annotation error in wait.h

sched: Fix docbook parameter annotation error in wait.h

Missing "@" in include/linux/wait.h causes "make htmldocs" to fail
with the following warning messages:

  Warning(/home/iida/Repo/linux-next//include/linux/wait.h:304):
  No description found for parameter 'cmd1'
  Warning(/home/iida/Repo/linux-next//include/linux/wait.h:304):
  No description found for parameter 'cmd2'

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Cc: trivial@kernel.org
Cc: peterz@infradead.org
Link: http://lkml.kernel.org/r/1390321326-23120-1-git-send-email-standby24x7@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>