.github/PULL_REQUEST_TEMPLATE.md: Fix link to contributors guide
.github/PULL_REQUEST_TEMPLATE.md: Fix link to contributors guide

The file was converted to Markdown and renamed appropriately in
2e07506a12e126894cd820304465162bc0e732b4.

CLA: trivial

Reviewed-by: Paul Dale <bc4a6b4cd7a8395c734c8eb5aef04a9c30188c85@oracle.com>
Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11223)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl