master -> main (#918)

README.md

@@ -1,10 +1,10 @@
 [![Build Status](https://travis-ci.org/grpc/grpc-swift.svg?branch=cgrpc)](https://travis-ci.org/grpc/grpc-swift)
 
-# ‼️ Please use the `master` branch ‼️
+# ‼️ Please use the `main` branch ‼️
 
 **gRPC Swift versions** `v0.x` **based on gRPC-Core will soon be replaced with a re-implementation based on [SwiftNIO](https://github.com/apple/swift-nio).**
 
-**We strongly suggest that new projects use [the re-implementation from the `master` branch](https://github.com/grpc/grpc-swift/tree/master) which we consider to be production ready.**
+**We strongly suggest that new projects use [the re-implementation from the `main` branch](https://github.com/grpc/grpc-swift/tree/main) which we consider to be production ready.**
 
 **Please see [DEPRECATION.md](DEPRECATION.md) for more information.**