sensors-and-actuators: Point to private branches for extensibility

Fix a broken link by pointing it to the page describing how to extend
the data with private branches.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

content/designs/sensors-and-actuators.md

@@ -896,7 +896,7 @@ which supports getting and setting properties, and being notified when
 they change.
 
 If an OEM, third party or application developer wishes to add new sensor
-or actuator types, they should follow the [extension process][w3-extending] and
+or actuator types, they should follow the [extension process][Private Branch] and
 request that the extensions be standardised by Apertis — they will then
 be released in the next version of the Sensors and Actuators API,
 available for all applications to use. If a vehicle needs to be released