Update the mention of master with unstable/v1

Author: webknjaz

README.md

@@ -29,10 +29,10 @@ To use the action add the following step to your workflow file (e.g.
     password: ${{ secrets.PYPI_API_TOKEN }}
 ```
 
-> **Pro tip**: instead of using branch pointers, like `master`, pin versions of
-Actions that you use to tagged versions or sha1 commit identifiers. This will
-make your workflows more secure and better reproducible, saving you from sudden
-and unpleasant surprises.
+> **Pro tip**: instead of using branch pointers, like `unstable/v1`, pin
+versions of Actions that you use to tagged versions or sha1 commit identifiers.
+This will make your workflows more secure and better reproducible, saving you
+from sudden and unpleasant surprises.
 
 A common use case is to upload packages only on a tagged commit, to do so add a
 filter to the step: