nrf/readme: Update make flash command when defining board.

Update the "make flash" command sample to include BOARD parameter
when building for a specific target board.

ports/nrf/README.md

@@ -54,7 +54,7 @@ By default, the PCA10040 (nrf52832) is used as compile target. To build and flas
 Alternatively the target board could be defined:
 
      make BOARD=pca10040
-     make flash
+     make BOARD=pca10040 flash
      
 ## Compile and Flash with Bluetooth Stack