xfrm: improve wording of comment above XFRM_OFFLOAD flags

I have noticed a few minor wording issues in a comment recently added
above XFRM_OFFLOAD flags in 7c76ecd9 ("xfrm: enforce validity of
offload input flags").

Signed-off-by: Petr Vaněk <arkamar@atlas.cz>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>