[readme] Fix branch name (master -> main)

Author: KonstHardy

CHANGELOG.md

@@ -89,4 +89,4 @@
 
 ## Before 15.5.0
 
-PropTypes was previously included in React, but is now a separate package. For earlier history of PropTypes [see the React change log.](https://github.com/facebook/react/blob/master/CHANGELOG.md)
+PropTypes was previously included in React, but is now a separate package. For earlier history of PropTypes [see the React change log.](https://github.com/facebook/react/blob/HEAD/CHANGELOG.md)

CONTRIBUTING.md

@@ -8,7 +8,7 @@ The code of conduct is described in [`CODE_OF_CONDUCT.md`](CODE_OF_CONDUCT.md).
 ## Pull Requests
 We actively welcome your pull requests.
 
-1. Fork the repo and create your branch from `master`.
+1. Fork the repo and create your branch from `main`.
 2. If you've added code that should be tested, add tests.
 3. If you've changed APIs, update the documentation.
 4. Ensure the test suite passes.

README.md

@@ -1,4 +1,4 @@
-# prop-types [![Build Status](https://travis-ci.com/facebook/prop-types.svg?branch=master)](https://travis-ci.org/facebook/prop-types)
+# prop-types [![Build Status](https://travis-ci.com/facebook/prop-types.svg?branch=main)](https://travis-ci.org/facebook/prop-types)
 
 Runtime type checking for React props and similar objects.
 
@@ -84,7 +84,7 @@ MyComponent.propTypes = {
 
   // A React element type (eg. MyComponent).
   // a function, string, or "element-like" object (eg. React.Fragment, Suspense, etc.)
-  // see https://github.com/facebook/react/blob/master/packages/shared/isValidElementType.js
+  // see https://github.com/facebook/react/blob/HEAD/packages/shared/isValidElementType.js
   optionalElementType: PropTypes.elementType,
 
   // You can also declare that a prop is an instance of a class. This uses