doc/manual: correct nginx reverse proxy example

- hydra does not remove the base URI from the request before processing
  it, so this must be done in the reverse proxy. in nginx this is done
  by giving proxy_pass a URI rather than a protocol/host/port; see:

  https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_pass

- proxy_redirect is not correct/required: hydra uses proxy headers to
  correctly form redirects in most cases, and where it doesn't it
  produces local redirects which aren't matched by this directive anyway

doc/manual/src/configuration.md

@@ -63,8 +63,7 @@ following:
         .. other configuration ..
         location /hydra/ {
 
-            proxy_pass     http://127.0.0.1:3000;
-            proxy_redirect http://127.0.0.1:3000 https://example.com/hydra;
+            proxy_pass http://127.0.0.1:3000/;
 
             proxy_set_header  Host              $host;
             proxy_set_header  X-Real-IP         $remote_addr;
@@ -74,6 +73,9 @@ following:
         }
     }
 
+Note the trailing slash on the `proxy_pass` directive, which causes nginx to
+strip off the `/hydra/` part of the URL before passing it to hydra.
+
 Populating a Cache
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