cgroup: trivial fixes for Documentation/cgroups/cgroups.txt

While reading through Documentation/cgroups/cgroups.txt, I found a
number of minor wordos and typos. The patch below is a conservative
handling of some of these: it provides just a number of "obviously
correct" fixes to the English that improve the readability
of the document somewhat. Obviously some more significant
fixes could be made to the document, but some of those
may not be in the "obviously correct" category.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>