docs: Fix wording in optional zram feature docs

This fixes some simple grammar errors in the documentation for zram,
specifically errors in the optional feature section of the zram
documentation.

Signed-off-by: Ethan Dye <mrtops03@gmail.com>
Link: https://lore.kernel.org/r/20220207235442.95090-1-mrtops03@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>