message: Update PR welcoming badge link
diff: <p align="center">
  <a href="#backers"><img src="https://opencollective.com/jest/backers/badge.svg" alt="Backers on Open Collective"></a>
  <a href="#sponsors"><img src="https://opencollective.com/jest/sponsors/badge.svg" alt="Sponsors on Open Collective"></a>
  - <a href="https://github.com/facebook/jest/pulls"><img src="https://img.shields.io/badge/PRs%20-welcome-brightgreen.svg" alt="PR's welcome"></a>
  + <a href="https://github.com/facebook/jest/blob/master/CONTRIBUTING.md"><img src="https://img.shields.io/badge/PRs%20-welcome-brightgreen.svg" alt="PR's welcome"></a>
  <a href="https://opensource.org/licenses/MIT"><img src="https://img.shields.io/badge/License-MIT-yellow.svg" alt="License: MIT"></a>
  <a href="https://snyk.io/test/github/facebook/jest?targetFile=packages/jest/package.json"><img src="https://snyk.io/test/github/facebook/jest/badge.svg?targetFile=packages/jest/package.json" alt="Known Vulnerabilities" data-canonical-src="https://snyk.io/test/github/facebook/jest?targetFile=packages/jest/package.json" style="max-width:100%;"></a>
  </p>
  