CONTRIBUTING.md: add missing "on"

"Guidelines how to" -> "Guidelines on how to"

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23488)

CONTRIBUTING.md

@@ -98,5 +98,5 @@ guidelines:
     with a specific release without having to sift through the higher
     noise ratio in git-log.
 
- 8. Guidelines how to integrate error output of new crypto library modules
+ 8. Guidelines on how to integrate error output of new crypto library modules
     can be found in [crypto/err/README.md](crypto/err/README.md).