doc: typo fix and sentence correction

Fix typo and improve sentence readability in documentation.

Signed-off-by: Jieqiang Wang <jieqiang.wang@arm.com>
Reviewed-by: Lijian Zhang <lijian.zhang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

Change-Id: I06ce3bb3527a8d924035f2bc0abfc8194111f92f