DATA_RATE vs DATARATE

Docs say on lines 128-137 of adafruit_lis3dh.py:
```
        Could have the following values:
        * DATA_RATE_400_HZ
        * DATA_RATE_200_HZ
```

but actual constants are:
```
DATARATE_400_HZ = const(0b0111)  # 400Hz
DATARATE_200_HZ = const(0b0110)  # 200Hz
```

The extra underscore should be removed.