Rename branch from master to main

Signed-off-by: Derek McGowan <derek@mcg.dev>

.github/workflows/ci.yml

@@ -2,9 +2,9 @@ name: CI
 
 on:
   push:
-    branches: [ master ]
+    branches: [ main ]
   pull_request:
-    branches: [ master ]
+    branches: [ main ]
 
 jobs:
 

README.md

@@ -1,7 +1,7 @@
 # ttrpc
 
 [![Build Status](https://github.com/containerd/ttrpc/workflows/CI/badge.svg)](https://github.com/containerd/ttrpc/actions?query=workflow%3ACI)
-[![codecov](https://codecov.io/gh/containerd/ttrpc/branch/master/graph/badge.svg)](https://codecov.io/gh/containerd/ttrpc)
+[![codecov](https://codecov.io/gh/containerd/ttrpc/branch/main/graph/badge.svg)](https://codecov.io/gh/containerd/ttrpc)
 
 GRPC for low-memory environments.
 
@@ -56,8 +56,8 @@ TODO:
 
 ttrpc is a containerd sub-project, licensed under the [Apache 2.0 license](./LICENSE).
 As a containerd sub-project, you will find the:
- * [Project governance](https://github.com/containerd/project/blob/master/GOVERNANCE.md),
- * [Maintainers](https://github.com/containerd/project/blob/master/MAINTAINERS),
- * and [Contributing guidelines](https://github.com/containerd/project/blob/master/CONTRIBUTING.md)
+ * [Project governance](https://github.com/containerd/project/blob/main/GOVERNANCE.md),
+ * [Maintainers](https://github.com/containerd/project/blob/main/MAINTAINERS),
+ * and [Contributing guidelines](https://github.com/containerd/project/blob/main/CONTRIBUTING.md)
 
 information in our [`containerd/project`](https://github.com/containerd/project) repository.