doc: Fix nordic target

Author: rymanluk

docs/mesh/sample.rst

@@ -10,7 +10,7 @@ no need for configuring public address.
 
     newt target create blemesh
     newt target set blemesh app=@apache-mynewt-nimble/apps/blemesh
-    newt target set blemesh bsp=@apache-mynewt-core/hw/bsp/nrf52840pdk
+    newt target set blemesh bsp=@apache-mynewt-core/hw/bsp/nordic_pca10056
     newt target set blemesh build_profile=optimized
     newt target set blemesh syscfg=BLE_MESH_PB_GATT=1:BLE_MESH_DEV_UUID='(uint8_t[16]){0x22, 0x20, 0}'