Adjust link to LICENSE file (#1210)

## What is this PR doing?
Commit eeb6b06 changed `LICENSE.md` to
`LICENSE`. This PR updates a location that still uses `LICENSE.md`.

packages/docs/site/docs/13-contributing/01-index.md

@@ -18,4 +18,4 @@ WordPress Playground is an open-source project and welcomes all contributors fro
 
 -   As with all WordPress projects, we want to ensure a welcoming environment for everyone. With that in mind, all contributors are expected to follow our [Code of Conduct](https://make.wordpress.org/handbook/community-code-of-conduct/).
 -   Contributors should review the [coding standards](./03-coding-standards.md).
--   You maintain copyright over any contribution you make. By submitting a pull request you agree to release that code under [WordPress Playground License](https://github.com/WordPress/wordpress-playground/blob/HEAD/LICENSE.md).
+-   You maintain copyright over any contribution you make. By submitting a pull request you agree to release that code under [WordPress Playground License](https://github.com/WordPress/wordpress-playground/blob/HEAD/LICENSE).