Update license file path in README (#9149)

This will indicate the file using the relative path, it works for both local and GitHub web markdown renders, no need to jump to web browsers for local reading.

README.md

@@ -70,4 +70,4 @@ See [adopters](https://www.jenkins.io/project/adopters/) for the list of Jenkins
 
 # License
 
-Jenkins is **licensed** under the **[MIT License](https://github.com/jenkinsci/jenkins/blob/master/LICENSE.txt)**.
+Jenkins is **licensed** under the **[MIT License](LICENSE.txt)**.