chore: update default branch for repo in README (#1152)

PR to update the default branch in the README.

### Checklist

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* [ ] Sister
[deploy-sourcegraph](https://github.com/sourcegraph/deploy-sourcegraph)
change:
* [ ] Sister
[customer-replica](https://github.com/sourcegraph/deploy-sourcegraph-docker-customer-replica-1)
change (if necessary, for any changes affecting pure-docker or
configuration):
* [ ] All images have a valid tag and SHA256 sum


### Test plan

N/A

Author: BolajiOlajide

README.md

@@ -6,7 +6,7 @@ This repository is the deployment reference for [deploying Sourcegraph with Dock
 
 > 🚨 IMPORTANT: When upgrading Sourcegraph, please check [upgrading docs](https://docs.sourcegraph.com/admin/updates/docker_compose) to check if any manual migrations are necessary.
 >
-> The `master` branch tracks development. Use the branch of this repository corresponding to the
+> The `main` branch tracks development. Use the branch of this repository corresponding to the
 > version of Sourcegraph you wish to deploy, e.g. `git checkout 3.19`.
 
 For product and [pricing](https://about.sourcegraph.com/pricing/) information, visit