docs(upgrade): use placeholder version in example

Author: jong-kyung

docs/guide/upgrade.md

@@ -16,7 +16,7 @@ You can upgrade both of them independently.
 ```bash
 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 <version>              # install a specific version
 vp upgrade --registry <registry>  # use a custom npm registry
 ```