DOC: Fix docstring examples for assert_frame_equal
This fixes the formatting in docstring examples for `pandas.util.testing.assert_frame_equal` so they will be displayed correctly. 

- [X] closes #26003



# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26004?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@181f972`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26004/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26004?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #26004   +/-   ##
=========================================
  Coverage          ?   91.81%           
=========================================
  Files             ?      175           
  Lines             ?    52551           
  Branches          ?        0           
=========================================
  Hits              ?    48252           
  Misses            ?     4299           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.38% <ø> (?)` | |
| #single | `40.72% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26004?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26004/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.62% <ø> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26004?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26004?src=pr&el=footer). Last update [181f972...0beaf13](https://codecov.io/gh/pandas-dev/pandas/pull/26004?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26004?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@181f972`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26004/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26004?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #26004   +/-   ##
=========================================
  Coverage          ?   91.81%           
=========================================
  Files             ?      175           
  Lines             ?    52551           
  Branches          ?        0           
=========================================
  Hits              ?    48252           
  Misses            ?     4299           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.38% <ø> (?)` | |
| #single | `40.72% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26004?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26004/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.62% <ø> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26004?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26004?src=pr&el=footer). Last update [181f972...0beaf13](https://codecov.io/gh/pandas-dev/pandas/pull/26004?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks!