[docs] Fixed broken link to RADIUS module in README

Author: nemesifier

README.rst

@@ -55,7 +55,7 @@ Other popular building blocks that are part of the OpenWISP ecosystem are:
 - `openwisp-firmware-upgrader
   <https://openwisp.io/docs/stable/firmware-upgrader/>`_: automated
   firmware upgrades (single devices or mass network upgrades)
-- `openwisp-radius <https://openwisp.io/docs/stable/user/radius.html>`_:
+- `openwisp-radius <https://openwisp.io/docs/stable/radius/>`_:
   based on FreeRADIUS, allows to implement network access authentication
   systems like 802.1x WPA2 Enterprise, captive portal authentication,
   Hotspot 2.0 (802.11u)