Documentation/vm/hmm.txt: typos and syntaxes fixes

This fix typos and syntaxes, thanks to Randy Dunlap for pointing them out
(they were all my faults).

Link: http://lkml.kernel.org/r/20180409151859.4713-1-jglisse@redhat.comSigned-off-by: Jérôme Glisse <jglisse@redhat.com>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>