docs/vm: Minor editorial changes in the THP and hugetlbfs

Some minor wording changes and typo corrections.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>