Update reverse proxy file name (#6590)

s/reverse_proxy.rst/reverse_proxy.md/

README.rst

@@ -393,4 +393,4 @@ something like the following in their logs::
     2019-09-11 19:32:04,271 - synapse.federation.transport.server - 288 - WARNING - GET-11752 - authenticate_request failed: 401: Invalid signature for server <server> with key ed25519:a_EqML: Unable to verify signature for <server>
 
 This is normally caused by a misconfiguration in your reverse-proxy. See
-`<docs/reverse_proxy.rst>`_ and double-check that your settings are correct.
+`<docs/reverse_proxy.md>`_ and double-check that your settings are correct.