Update dialog heading for clarity

Author: microbit-matt-hillsdon

src/messages/ui.en.json

@@ -186,7 +186,7 @@
   "connectMB.usbDownloading.header": "Downloading program to micro:bit",
   "connectMB.usbDownloading.subtitle": "Please wait. Downloading program to micro:bit.",
 
-  "connectMB.connectBattery.heading": "Connect battery pack",
+  "connectMB.connectBattery.heading": "Connect battery pack and disconnect USB",
   "connectMB.connectBattery.subtitle": "Disconnect the micro:bit from the computer and connect the battery pack.",
 
   "connectMB.pattern.heading": "Copy pattern",