mfd: arizona: Correct link for sound binding document

Signed-off-by: Charles Keepax <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/mfd/arizona.txt

···
 Also see child specific device properties:
   Regulator - ../regulator/arizona-regulator.txt
   Extcon    - ../extcon/extcon-arizona.txt
-  Sound     - ../sound/arizona.txt
+  Sound     - ../sound/wlf,arizona.txt
 
 Example: