doc: update tls Caddy directive set in tls.md

This configuration will not result in a certificate usage error for host php:80.

Author: 7-zete-7

docs/tls.md

@@ -30,7 +30,10 @@ For instance, to use self-signed certificates created with [mkcert](https://gith
     ```diff
     php:
       environment:
-    +    CADDY_SERVER_EXTRA_DIRECTIVES: "tls /etc/caddy/certs/tls.pem /etc/caddy/certs/tls.key"
+    +    CADDY_EXTRA_CONFIG: |
+    +      https:// {
+    +        tls /etc/caddy/certs/tls.pem /etc/caddy/certs/tls.key
+    +      }
         # ...
       volumes:
     +    - ./frankenphp/certs:/etc/caddy/certs:ro