firmware: fw_update: change board name to for Zephyr 3.7

Update the board name to use slashes instead of underscores.

Signed-off-by: Mike Szczys <mike@golioth.io>

Author: szczys

docs/firmware/golioth-firmware-sdk/firmware-upgrade/fw-update-zephyr.md

@@ -51,7 +51,7 @@ Console](https://console.golioth.io).
 ### 2. Initial build and flash
 
 ```console
-west build -b nrf9160dk_nrf9160_ns examples/zephyr/fw_update
+west build -b nrf9160dk/nrf9160/ns examples/zephyr/fw_update
 west flash
 ```