Fix typo in TLS filenames

Signed-off-by: Jurrie Overgoor <1213142+Jurrie@users.noreply.github.com>

docker/README.md

@@ -57,8 +57,8 @@ configuration file there. Multiple application services are supported.
 Synapse requires a valid TLS certificate. You can do one of the following:
 
  * Provide your own certificate and key (as
-   `${DATA_PATH}/${SYNAPSE_SERVER_NAME}.crt` and
-   `${DATA_PATH}/${SYNAPSE_SERVER_NAME}.key`, or elsewhere by providing an
+   `${DATA_PATH}/${SYNAPSE_SERVER_NAME}.tls.crt` and
+   `${DATA_PATH}/${SYNAPSE_SERVER_NAME}.tls.key`, or elsewhere by providing an
    entire config as `${SYNAPSE_CONFIG_PATH}`).
 
  * Use a reverse proxy to terminate incoming TLS, and forward the plain http