docs(upgrade): drop dist-tag upgrade example

Author: jong-kyung

docs/guide/upgrade.md

@@ -17,7 +17,6 @@ You can upgrade both of them independently.
 vp upgrade                        # upgrade to the latest version
 vp upgrade --check                # check for updates without installing
 vp upgrade 0.2.0                  # install a specific version
-vp upgrade --tag alpha            # install from a dist-tag
 vp upgrade --registry <registry>  # use a custom npm registry
 ```