Update config.md (#1290)

Fix example that doesn't work with the currently used Caddy version

Author: skeemer

docs/config.md

@@ -202,7 +202,9 @@ This is an opt-in configuration that needs to be added to the global options in
 > This can also be configured using the `CADDY_GLOBAL_OPTIONS` environment config:
 
 ```sh
-CADDY_GLOBAL_OPTIONS="servers { enable_full_duplex }"
+CADDY_GLOBAL_OPTIONS="servers {
+  enable_full_duplex
+}"
 ```
 
 You can find more information about this setting in the [Caddy documentation](https://caddyserver.com/docs/caddyfile/options#enable-full-duplex).