Do not expose Admin API in caddy reverse proxy example (#16027)

Signed-off-by: Nils ANDRÉ-CHANG <nils@nilsand.re>

changelog.d/16027.doc

+Do not expose Admin API in caddy reverse proxy example. Contributed by @NilsIrl.

docs/reverse_proxy.md

@@ -95,7 +95,7 @@ matrix.example.com {
 }
 
 example.com:8448 {
-  reverse_proxy localhost:8008
+  reverse_proxy /_matrix/* localhost:8008
 }
 ```