Update example in HeartRateService.md

Point example to latest version.

Author: Amanda Butler

docs/api/bluetooth/HeartRateService.md

@@ -14,7 +14,7 @@ The HeartRateService class implements the Bluetooth Heart Rate service as define
 
 ### HeartRateService example
 
-[![View code](https://www.mbed.com/embed/?url=https://os.mbed.com/teams/mbed-os-examples/code/mbed-os-example-ble-HeartRate/)](https://os.mbed.com/teams/mbed-os-examples/code/mbed-os-example-ble-HeartRate/file/55c859f4efe0/source/main.cpp)
+[![View code](https://www.mbed.com/embed/?url=https://os.mbed.com/teams/mbed-os-examples/code/mbed-os-example-ble-HeartRate/)](https://os.mbed.com/teams/mbed-os-examples/code/mbed-os-example-ble-HeartRate/file/f5279652302f/source/main.cpp)
 
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