Update README.md

Author: wilr

README.md

@@ -3,7 +3,7 @@
 Adds a `DropdownImageField` field which enables you to display images alongside the captions.
 Uses a plugin for Chosen.js (which is used by SS), [Image-Select](https://github.com/websemantics/Image-Select). The plugin is modified.
 
-![Working screenshot](https://github.com/fullscreeninteractive/silverstripe-dropdownimagefield/raw/master/docs/img/ss.png)
+![Working screenshot](https://github.com/fullscreeninteractive/silverstripe-dropdownimagefield/raw/main/docs/img/ss.png)
 
 ## Requirements