message: fix link to classic yarn's `yarn link`
diff: @@ -160,8 +160,8 @@ yarn link matrix-js-sdk
  yarn install
  ```
  -See the [help for `yarn link`](https://yarnpkg.com/docs/cli/link) for more
  -details about this.
  +See the [help for `yarn link`](https://classic.yarnpkg.com/docs/cli/link) for
  +more details about this.
  Running tests
  =============
  