Replace `contributors guide` link in pull request template

[dev-jonghoonpark]

In the pull request template, the contributors guide link appears to be broken.

current link: https://github.com/apache/incubator-kie-kogito-runtimes#contributing-to-kogito

I suggest updating it to one of the following:

https://github.com/apache/incubator-kie-kogito-runtimes#building-and-contributing-to-kogito
or
https://github.com/apache/incubator-kie-kogito-runtimes/blob/main/CONTRIBUTING.md