docs: s/master/main/g in contributing docs

docs/development.rst

@@ -129,7 +129,7 @@ Contributing
 Submitting pull requests
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
-Submit pull requests (PRs) against the ``master`` branch, providing a good description of what you're doing and why. You must
+Submit pull requests (PRs) against the ``main`` branch, providing a good description of what you're doing and why. You must
 have legal permission to distribute any code you contribute to tox and it must be available under the MIT
 License. Provide tests that cover your changes and run the tests locally first. tox
 :ref:`supports <compatibility-requirements>` multiple Python versions and operating systems. Any pull request must
@@ -148,7 +148,7 @@ or whitespace within lines. Such changes can be made separately, as a "formattin
 Automated testing
 ~~~~~~~~~~~~~~~~~
 
-All pull requests and merges to the ``master`` branch are tested using :gh:`GitHub Actions <features/actions>`
+All pull requests and merges to the ``main`` branch are tested using :gh:`GitHub Actions <features/actions>`
 (configured by ``check.yml`` file inside the ``.github/workflows`` directory). You can find the status and the results
 to the CI runs for your PR on GitHub's Web UI for the pull request. You can also find links to the CI services' pages
 for the specific builds in the form of "Details" links, in case the CI run fails and you wish to view the output.