Update badge with main (#52)

Author: v1v

README.md

@@ -1,3 +1,3 @@
-[![Build Status](https://beats-ci.elastic.co/job/Library/job/go-txfile-mbp/job/master/badge/icon)](https://beats-ci.elastic.co/job/Library/job/go-txfile-mbp/job/master/)
+[![Build Status](https://beats-ci.elastic.co/job/Library/job/go-txfile-mbp/job/main/badge/icon)](https://beats-ci.elastic.co/job/Library/job/go-txfile-mbp/job/main/)
 
-# txfile
+# txfile