Fix name of mbed shell example. (#11391)

This makes the following work:

```
./scripts/build/build_examples.py --target-glob 'mbed*' build
--copy-artifacts-to out/artifacts
```

scripts/build/builders/mbed.py

@@ -56,7 +56,7 @@ def AppNamePrefix(self):
         elif self == MbedApp.PIGWEED:
             return 'chip-mbed-pigweed-app-example'
         elif self == MbedApp.SHELL:
-            return 'shell'
+            return 'chip-mbed-shell-example'
         else:
             raise Exception('Unknown app type: %r' % self)