board/broadcom/northstar: Fix defconfig make target

The readme.txt contains a make target that does not match the actual
defconfig file name, fix that.

Fixes: 1500b7d5c8 ("configs/broadcom_northstar: new defconfig")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

board/broadcom/northstar/readme.txt

@@ -25,7 +25,7 @@ device setup).
 How to build it
 ===============
 
-  $ make broadcom_ns_defconfig
+  $ make broadcom_northstar_defconfig
 
   $ make