update ngnix reverse-proxy example (#11680)

this should not be a case-insensitive match.

changelog.d/11680.doc

+Correct the documentation for `nginx` to use a case-sensitive url pattern. Fixes an error introduced in v1.21.0.

docs/reverse_proxy.md

@@ -63,7 +63,7 @@ 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.