Fix documentation and key error in calibration (#2633)

* Fix documentation and key error in calibration

- Tried to retrieve historical calibration and it was broken.
- Fixed docs to my understanding and fixed key error.
- Please verify that this is correct now per the API.
- Fix units in documentation.

cirq/google/engine/engine.py

@@ -737,15 +737,16 @@ def get_calibration(self, calibration_name: str) -> calibration.Calibration:
 
         Params:
             calibration_name: A string of the form
-                `<processor name>/calibrations/<ms since epoch>`
+                `projects/<project_id>/processors/<processor id>`
+                `/calibrations/<timestamp in seconds since epoch>`
 
         Returns:
             A dictionary containing the metadata.
         """
         response = self._make_request(
             self.service.projects().processors().calibrations().get(
                 name=calibration_name))
-        return calibration.Calibration(response['data']['data'])
+        return calibration.Calibration(response['data'])
 
     def sampler(self, processor_id: Union[str, List[str]],
                 gate_set: serializable_gate_set.SerializableGateSet

cirq/google/engine/engine_test.py

@@ -1037,7 +1037,7 @@ def test_calibration_from_job(build):
         },
     }
     calibrations = service.projects().processors().calibrations()
-    calibrations.get().execute.return_value = {'data': _CALIBRATION}
+    calibrations.get().execute.return_value = _CALIBRATION
 
     engine = cg.Engine(project_id='project-id')
     job = engine.run_sweep(