DOC: Broken import in snippet (pandas.testing.assert_frame_equal)
### 

- [X] I have checked that the issue still exists on the latest versions of the docs on `master` [here](https://pandas.pydata.org/docs/dev/)


### Location of the documentation

 I have checked that the issue still exists on the latest versions of the docs on master: https://pandas.pydata.org/docs/dev/reference/api/pandas.testing.assert_frame_equal.html
 
 "pandas.testing.assert_frame_equal"

### Documentation problem

Example snippet for `assert_frame_equal` has a leading underscore in the import statement (for testing). I think this is unnecessary and breaks execution when attempted by a user. 

```
from pandas._testing import assert_frame_equal
df1 = pd.DataFrame({'a': [1, 2], 'b': [3, 4]})
df2 = pd.DataFrame({'a': [1, 2], 'b': [3.0, 4.0]})
```

### Suggested fix for documentation

Remove the underscore in question:

```
from pandas.testing import assert_frame_equal
df1 = pd.DataFrame({'a': [1, 2], 'b': [3, 4]})
df2 = pd.DataFrame({'a': [1, 2], 'b': [3.0, 4.0]})
```

This should fix execution. 