Update example in BatteryService.md

Point example to latest version.

Author: Amanda Butler

docs/api/bluetooth/BatteryService.md

@@ -12,7 +12,7 @@ The BatteryService class implements the Bluetooth Battery Service as defined by
 
 ### BatteryService example
 
-[![View code](https://www.mbed.com/embed/?url=https://os.mbed.com/teams/mbed-os-examples/code/mbed-os-example-ble-BatteryLevel/)](https://os.mbed.com/teams/mbed-os-examples/code/mbed-os-example-ble-BatteryLevel/file/5cf292205001/source/main.cpp)
+[![View code](https://www.mbed.com/embed/?url=https://os.mbed.com/teams/mbed-os-examples/code/mbed-os-example-ble-BatteryLevel/)](https://os.mbed.com/teams/mbed-os-examples/code/mbed-os-example-ble-BatteryLevel/file/15a6111a679e/source/main.cpp)
 
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