Update get_started.md (#11054)
Update get_started.md (#11054)

Add defination of eval_input_fn in example.

# train
estimator.fit(input_fn=input_fn, steps=1000)
# Here we evaluate how well our model did. 
train_loss = estimator.evaluate(input_fn=input_fn)
eval_loss = estimator.evaluate(input_fn=eval_input_fn)
```
The `eval_input_fn` is not defined here.
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