Minor change

Spelling mistake
listning -> listening

config-example.yaml

@@ -164,7 +164,7 @@ tls_letsencrypt_cache_dir: /var/lib/headscale/cache
 # See [docs/tls.md](docs/tls.md) for more information
 tls_letsencrypt_challenge_type: HTTP-01
 # When HTTP-01 challenge is chosen, letsencrypt must set up a
-# verification endpoint, and it will be listning on:
+# verification endpoint, and it will be listening on:
 # :http = port 80
 tls_letsencrypt_listen: ":http"