Fix a link in `README.rst` (#12495)

* Fix a link in `README.rst`

* newsfile

README.rst

@@ -312,7 +312,7 @@ We recommend using the demo which starts 3 federated instances running on ports
 
 (to stop, you can use `./demo/stop.sh`)
 
-See the [demo documentation](https://matrix-org.github.io/synapse/develop/development/demo.html)
+See the `demo documentation <https://matrix-org.github.io/synapse/develop/development/demo.html>`_
 for more information.
 
 If you just want to start a single instance of the app and run it directly::

changelog.d/12495.doc

@@ -0,0 +1 @@
+Fix a broken link in `README.rst`.