Use full path for uninstall script in upgrade instructions

Makes the upgrade section consistent with the uninstall section by
using the full path /usr/local/bin/uninstall-container.sh instead of
just uninstall-container.sh.

Author: claudeaceae

README.md

@@ -22,7 +22,7 @@ If you're upgrading, first stop and uninstall your existing `container` (the `-k
 
 ```bash
 container system stop
-uninstall-container.sh -k
+/usr/local/bin/uninstall-container.sh -k
 ```
 
 Download the latest signed installer package for `container` from the [GitHub release page](https://github.com/apple/container/releases).