README.md incorrect example

Hi,

The example should read:

```python
import time
from mpu9250 import MPU9250

sensor = MPU9250()

print("MPU9250 id: " + hex(sensor.read_whoami()))

while True:
    print(sensor.read_acceleration())
    print(sensor.read_gyro())
    print(sensor.read_magnetic())

    time.sleep(1000)
```