ENH: Change master->main for LICENSE

Author: hjmjohnson

README.rst

@@ -8,7 +8,7 @@ ITKTwoProjectionRegistration
     :alt: PyPI Version
 
 .. image:: https://img.shields.io/badge/License-Apache%202.0-blue.svg
-    :target: https://github.com/InsightSoftwareConsortium/TwoProjectionRegistration/blob/master/LICENSE)
+    :target: https://github.com/InsightSoftwareConsortium/TwoProjectionRegistration/blob/main/LICENSE)
     :alt: License
 
 Overview