Update reverse_proxy.rst (#5397)

Updates reverse_proxy.rst with information about nginx' URI normalisation.

changelog.d/5397.doc

@@ -0,0 +1 @@
+Add information about nginx normalisation to reverse_proxy.rst. Contributed by @skalarproduktraum - thanks!

docs/reverse_proxy.rst

@@ -48,6 +48,8 @@ Let's assume that we expect clients to connect to our server at
               proxy_set_header X-Forwarded-For $remote_addr;
           }
       }
+      
+  Do not add a `/` after the port in `proxy_pass`, otherwise nginx will canonicalise/normalise the URI.
 
 * Caddy::