Update PULL_REQUEST_TEMPLATE.md

[reedy]

• Remove excess newlines
• Fix path to CONTRIBUTING.md

Summary

Fixing minor issues in pull request template.

Fixes https://github.com/slackhq/goSDL/issues/28

Requirements (place an x in each [ ])

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've written tests to cover the new code and functionality included in this PR.
• [x] I've read, agree to, and signed the Contributor License Agreement (CLA).