CHANGES.md: move SSL_CTX_is_server() entry to the 4.0 section

CHANGES.md: move SSL_CTX_is_server() entry to the 4.0 section

Also reword it to match the style of other entries.

Complements: ca20e54e8674 "SSL_CTX_is_server() was added."
Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Paul Yang <paulyang.inf@gmail.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Tomas Mraz <tomas@openssl.foundation>
MergeDate: Wed Apr  8 10:00:05 2026
(Merged from https://github.com/openssl/openssl/pull/30686)

(cherry picked from commit 4c189a095412822d43aec66abbd3672c76f5d226)