build: clean up references to old master branch

Cleans up all references to the old `master` branch.

Author: devversion

material.angular.io/README.md

@@ -28,9 +28,9 @@ issues with rendering or layout of the documentation pages, etc.
 The guides, examples, and docs content repo
 [angular/material2-docs-content](https://github.com/angular/material2-docs-content) contains the
 documentation content and examples. They are generated from:
-- [Angular Material and CDK Guides](https://github.com/angular/components/tree/master/guides)
-- [Material components, services, and directives](https://github.com/angular/components/tree/master/src/material)
-- [CDK components, services, and directives](https://github.com/angular/components/tree/master/src/cdk)
+- [Angular Material and CDK Guides](https://github.com/angular/components/tree/main/guides)
+- [Material components, services, and directives](https://github.com/angular/components/tree/main/src/material)
+- [CDK components, services, and directives](https://github.com/angular/components/tree/main/src/cdk)
 
 ## Development Setup
 

material.angular.io/src/app/shared/footer/footer.html

@@ -19,7 +19,7 @@
     <div class="docs-footer-copyright">
       <div>
         <span>Powered by Google LLC ©2010-2021.</span>
-        <a href="https://github.com/angular/components/blob/master/LICENSE">Code licensed under an MIT-style License.</a>
+        <a href="https://github.com/angular/components/blob/main/LICENSE">Code licensed under an MIT-style License.</a>
         <span>Documentation licensed under CC BY 4.0.</span>
       </div>
     </div>