Fix metadata export command in README

* the extension in use is `.yaml` rather than `.yml`

README.md

@@ -37,7 +37,7 @@ The SAML backend endpoints are available under the path `/saml`.
 ## SAML service provider metadata
 
 The SAML service provider metadata can be generated with the following command:
-`pipenv run satosa-saml-metadata proxy_conf.yml ssl/<tls-key-file> ssl/<tls-cert-file>`
+`pipenv run satosa-saml-metadata proxy_conf.yaml ssl/<tls-key-file> ssl/<tls-cert-file>`
 
 If the operation was successful, then the generated should be stored in `backend.xml`.