[openssl/openssl] ea1550: CONTRIBUTING.md: add missing "on"

[openssl-machine]

  Branch: refs/heads/master
  Home:   https://github.com/openssl/openssl
  Commit: ea15508b4ffa77d440e3f2c817998271bfeb3b9c
      https://github.com/openssl/openssl/commit/ea15508b4ffa77d440e3f2c817998271bfeb3b9c
  Author: James Muir <james at openssl.org>
  Date:   2024-02-07 (Wed, 07 Feb 2024)

  Changed paths:
    M CONTRIBUTING.md

  Log Message:
  -----------
  CONTRIBUTING.md: add missing "on"

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

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