Update example in CellularInterface.md

Point example to latest version.

Author: Amanda Butler

docs/api/networkinterfaces/CellularInterface.md

@@ -70,7 +70,7 @@ To bring up the network interface:
 
 This example establishes connection with the cellular network using Mbed OS CellularInterface.
 
-[![View code](https://www.mbed.com/embed/?url=https://os.mbed.com/teams/mbed-os-examples/code/mbed-os-example-cellular/)](https://os.mbed.com/teams/mbed-os-examples/code/mbed-os-example-cellular/file/cf737b827270/main.cpp)
+[![View code](https://www.mbed.com/embed/?url=https://github.com/ARMmbed/mbed-os-example-cellular)](https://github.com/ARMmbed/mbed-os-example-cellular/blob/master/main.cpp)
 
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