Update reverse_proxy.md

In issue matrix-org#11895, the redirect from /_matrix/client to the URL configured in web_client_location was moved to the root URL (/). This change requires local administrators to update their proxy setups to include the root URL, as the current recommendations only include proxies for /_matrix/ and /_synapse/client/. This is necessary for the new location of the redirect to be effective.

This issue was raised in matrix-org#14722 by @sumpfralle

docs/reverse_proxy.md

@@ -67,7 +67,7 @@ server {
 
     server_name matrix.example.com;
 
-    location ~ ^(/_matrix|/_synapse/client) {
+    location ~ ^(/_matrix|/_synapse/client|/$) {
         # note: do not add a path (even a single /) after the port in `proxy_pass`,
         # otherwise nginx will canonicalise the URI and cause signature verification
         # errors.