readme: Added missing directory change in build instructions

The current build instructions omit a necessary directory
change after creating the workspace.

Signed-off-by: Andy Sinclair <andy.sinclair@nordicsemi.no>

Author: aasinclair

README.md

@@ -57,6 +57,7 @@ west update
 To build the application, run the following command:
 
 ```shell
+cd example-application
 west build -b $BOARD app
 ```