Update runtime check command (#721)

Co-authored-by: Pat Connors <[email protected]>

docs/guide/codecs.md

@@ -61,7 +61,7 @@ This command should return the version(s) of the runtime that you installed.
 To check what runtime version a certain program is currently running (i.e Firefox), enter in a terminal:
 
 ```bash
-flatpak info org.mozilla.firefox | grep Runtime
+flatpak info --show-extensions org.mozilla.firefox | grep runtime
 ```
 
 Ensure that you also get the ffmpeg-full version returned by the previous command.