Add group delete to uninstall instructions (#1924)

After executing the `userdel ollama` command, I saw this message:

```sh
$ sudo userdel ollama
userdel: group ollama not removed because it has other members.
```

Which reminded me that I had to remove the dangling group too. For completeness, the uninstall instructions should do this too.

Thanks!

docs/linux.md

@@ -109,8 +109,9 @@ Remove the ollama binary from your bin directory (either `/usr/local/bin`, `/usr
 sudo rm $(which ollama)
 ```
 
-Remove the downloaded models and Ollama service user:
+Remove the downloaded models and Ollama service user and group:
 ```bash
 sudo rm -r /usr/share/ollama
 sudo userdel ollama
+sudo groupdel ollama
 ```