Fix typo in CONTRIBUTING and README

* Fix `master` to `main` in the README

Signed-off-by: Tran Ngoc Nhan <[email protected]>

Author: ngocnhan-tran1996

README.md

@@ -5,7 +5,7 @@ Spring for Apache Kafka
 
 # Code of Conduct
 
-Please see our [Code of conduct](https://github.com/spring-projects/.github/blob/master/CODE_OF_CONDUCT.md).
+Please see our [Code of conduct](https://github.com/spring-projects/.github/blob/main/CODE_OF_CONDUCT.md).
 
 # Reporting Security Vulnerabilities