Update CI and README.md to use the main branch (#195)

* Update CI and README.md to use the main branch

* Update link

Author: esteve

.github/workflows/rust.yml

@@ -2,9 +2,9 @@ name: Rust
 
 on:
   push:
-    branches: [ master ]
+    branches: [ main ]
   pull_request:
-    branches: [ master ]
+    branches: [ main ]
 
 env:
   CARGO_TERM_COLOR: always

README.md

@@ -3,7 +3,7 @@ ROS 2 for Rust
 
 | Target | Status |
 |----------|--------|
-| **Ubuntu 20.04** | [![Build Status](https://github.com/ros2-rust/ros2_rust/actions/workflows/rust.yml/badge.svg?branch=master)](https://github.com/ros2-rust/ros2_rust/actions/workflows/rust.yml?branch=master) |
+| **Ubuntu 20.04** | [![Build Status](https://github.com/ros2-rust/ros2_rust/actions/workflows/rust.yml/badge.svg?branch=main)](https://github.com/ros2-rust/ros2_rust/actions/workflows/rust.yml?branch=main) |
 
 Introduction
 ------------

rclrs/src/lib.rs

@@ -3,7 +3,7 @@
 //!
 //! For getting started, see the [README][1].
 //!
-//! [1]: https://github.com/ros2-rust/ros2_rust/blob/master/README.md
+//! [1]: https://github.com/ros2-rust/ros2_rust/blob/main/README.md
 
 mod context;
 mod error;