document the -no_ecdhe option in s_server man page

the option is provided in the -help message of the s_server utility
but it is not documented in the man page, this fixes it

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
(Merged from https://github.com/openssl/openssl/pull/7046)