DOC: Fix docstring for assert_frame_equal examples
#### Problem description

The docstring for assert_frame_equals examples is not formatted correctly.

The example section of the docstring for `pandas.util.testing.assert_frame_equal` is not formatted correctly and causes them to be displayed wrong. There needs to be some added newlines to separate the code blocks.

```
This example shows comparing two DataFrames that are equal but with columns of differing dtypes.

>>> from pandas.util.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]})

df1 equals itself. >>> assert_frame_equal(df1, df1)

df1 differs from df2 as column ‘b’ is of a different type. >>> assert_frame_equal(df1, df2) Traceback (most recent call last): AssertionError: Attributes are different

Attribute “dtype” are different [left]: int64 [right]: float64

Ignore differing dtypes in columns with check_dtype. >>> assert_frame_equal(df1, df2, check_dtype=False)
```
see https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.testing.assert_frame_equal.html#pandas-testing-assert-frame-equal
