Update README for v6.3

Currently it has examples for v6, which are not compatible with v6.3:

```
FlatpakUnexpected input(s) 'bundle', 'manifest-path', 'cache-key', valid inputs are ['entryPoint', 'args']
```

EDIT: looks like it's a simple bump, but I was following the suggested path by GitHub and it removed the /flatpak-builder part, which is not how this action is used.