Change legacy-v2 sheild label

Author: dpvc

README.md

@@ -1,5 +1,5 @@
 # MathJax (Source Repository)
-<img class="shield" alt="GitHub release version" src="https://img.shields.io/github/v/release/mathjax/MathJax-src?sort=semver"> <img class="shield" alt="GitHub release version (branch)" src="https://img.shields.io/github/package-json/v/mathjax/MathJax/legacy-v2">
+<img class="shield" alt="GitHub release version" src="https://img.shields.io/github/v/release/mathjax/MathJax-src?sort=semver"> <img class="shield" alt="GitHub release version (branch)" src="https://img.shields.io/github/package-json/v/mathjax/MathJax/legacy-v2?label=release v2">
 
 ## Beautiful math in all browsers