Minor WINDOWS.md cleanups

Minor WINDOWS.md cleanups

The possessive form of "Windows" has been updated from "Windows's"
to "Windows'".

The function call "a poll(2) call" has been specified as
"a poll(2) system call" for clarity.

The phrase "and supposed" has been corrected to "and was supposed" to
improve sentence structure.

The phrase "However Microsoft has" now includes a comma, revised to
"However, Microsoft has" to enhance readability.

The statement "Supporting these is a pain" has been adjusted to
"Supporting these can be a pain" to better convey potential variability
in user experience.

CLA: trivial

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Yang <kaishen.yy@antfin.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24242)