chore(python): rename default branch to main (#935)

Source-Link: googleapis/synthtool@5c0fa62
Post-Processor: gcr.io/repo-automation-bots/owlbot-python:latest@sha256:0ffe3bdd6c7159692df5f7744da74e5ef19966288a6bf76023e8e04e0c424d7d

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Tim Swast <swast@google.com>
Co-authored-by: Peter Lamut <plamut@users.noreply.github.com>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>

.github/.OwlBot.lock.yaml

@@ -1,3 +1,3 @@
 docker:
   image: gcr.io/repo-automation-bots/owlbot-python:latest
-  digest: sha256:9743664022bd63a8084be67f144898314c7ca12f0a03e422ac17c733c129d803
+  digest: sha256:0ffe3bdd6c7159692df5f7744da74e5ef19966288a6bf76023e8e04e0c424d7d

CONTRIBUTING.rst

@@ -113,9 +113,9 @@ Coding Style
    export GOOGLE_CLOUD_TESTING_BRANCH="main"
 
   By doing this, you are specifying the location of the most up-to-date
-  version of ``python-bigquery``. The the suggested remote name ``upstream``
-  should point to the official ``googleapis`` checkout and the
-  the branch should be the main branch on that remote (``main``).
+  version of ``python-bigquery``. The
+  remote name ``upstream`` should point to the official ``googleapis``
+  checkout and the branch should be the default branch on that remote (``main``).
 
 - This repository contains configuration for the
   `pre-commit <https://pre-commit.com/>`__ tool, which automates checking