Fix branch in release instructions (#388)

## Description

<!-- Provide a brief description of the PR's purpose here. -->

Small edit in CONTRIBUTING.md.

## TODO

<!-- Notable points that this PR has either accomplished or will
accomplish. -->

## Questions

<!-- Any concerns or points of confusion? -->

## Status

- [x] I have read the guidelines in

[CONTRIBUTING.md](https://github.com/icaros-usc/pyribs/blob/master/CONTRIBUTING.md)
- [x] I have formatted my code using `yapf`
- [x] I have tested my code by running `pytest`
- [x] I have linted my code with `pylint`
- [N/A] I have added a one-line description of my change to the
changelog in
      `HISTORY.md`
- [x] This PR is ready to go

CONTRIBUTING.md

@@ -235,7 +235,7 @@ from `master` into the corresponding release branch.
       ```
    1. Add all necessary info on the version to `HISTORY.md`.
 1. Once the PR above has been merged, checkout the release branch for the
-   corresponding minor version, e.g., for `0.6.2`, check out `release/0.6.0`.
+   corresponding minor version, e.g., for `0.6.2`, check out `release/0.6.x`.
 1. On the release branch, cherry-pick the commit for the PR you just created.
    Also cherry pick any other bug fixes which need to be released in this patch.
 1. On the release branch, edit `HISTORY.md` to remove any irrelevant history,