Doc: net: dsa: b53: update location of referenced dsa.txt

The referenced file dsa.txt is located at
Documentation/devicetree/bindings/net/dsa/dsa.txt
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/dsa/b53.txt

@@ -34,7 +34,7 @@ Required properties:
       "brcm,bcm6328-switch"
       "brcm,bcm6368-switch" and the mandatory "brcm,bcm63xx-switch"
 
-See Documentation/devicetree/bindings/dsa/dsa.txt for a list of additional
+See Documentation/devicetree/bindings/net/dsa/dsa.txt for a list of additional
 required and optional properties.
 
 Examples: