Update branch name

Author: sebastianbergmann

.github/PULL_REQUEST_TEMPLATE/IMPROVEMENT.md

@@ -5,5 +5,5 @@ labels: type/enhancement
 ---
 
 <!--
-- Please target the master branch of PHPUnit.
+- Please target the main branch of PHPUnit.
 -->

.github/PULL_REQUEST_TEMPLATE/NEW_FEATURE.md

@@ -5,5 +5,5 @@ labels: type/enhancement
 ---
 
 <!--
-- Please target the master branch of PHPUnit.
+- Please target the main branch of PHPUnit.
 -->

README.md

@@ -25,7 +25,7 @@ Alternatively, you may use [Composer](https://getcomposer.org/) to download and
 
 ## Contribute
 
-Please refer to [CONTRIBUTING.md](https://github.com/sebastianbergmann/phpunit/blob/master/.github/CONTRIBUTING.md) for information on how to contribute to PHPUnit and its related projects.
+Please refer to [CONTRIBUTING.md](https://github.com/sebastianbergmann/phpunit/blob/main/.github/CONTRIBUTING.md) for information on how to contribute to PHPUnit and its related projects.
 
 ## List of Contributors