sched/completions/Documentation: Fix a couple of punctuation nits

sched/completions/Documentation: Fix a couple of punctuation nits

This patch fixes a couple of punctuation nits which can make the document
more correct and readable.

Also missing "()" are added to some function references for consistency.

Signed-off-by: John Garry <john.garry@huawei.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: corbet@lwn.net
Cc: linux-doc@vger.kernel.org
Link: http://lkml.kernel.org/r/1539183392-239389-1-git-send-email-john.garry@huawei.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>