docs/scheduler: fix typo and warning in sched-bwc

docs/scheduler: fix typo and warning in sched-bwc

a) since d73df887b6b8 ("sched/fair: Add document for burstable CFS bandwidth")
[cpu.cfs_quota_us: the total available run-time within a period (in] shoud be removed,
let's delete it.

b) Add a period.

c) fix a build warning:

   linux-next/Documentation/scheduler/sched-bwc.rst:243: WARNING: Inline emphasis
   start-string without end-string.

Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Reviewed-by: Alex Shi <alexs@kernel.org>
Link: https://lore.kernel.org/r/163a4dde20b8c4b68d668977a668e281d18fcf92.1638517064.git.siyanteng@loongson.cn
Signed-off-by: Jonathan Corbet <corbet@lwn.net>