docs/llvmpipe.html: Minor edits

Language and spelling fixups in three places.

Cc: "17.2" "17.3" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Ben Crocker <bcrocker@redhat.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

[Eric: move two fixes from the other patch to this one.]
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
(cherry picked from commit b43daf7b)