TST: Fix a doctest in style_render.py
Fix the following doctest in style_render.py file:
```
pandas/io/formats/style_render.py F

=================================== FAILURES ===================================
________ [doctest] pandas.io.formats.style_render.StylerRenderer.format ________
647 
648         >>> df = pd.DataFrame([[np.nan, 1.0, 'A'], [2.0, np.nan, 3.0]])
649         >>> df.style.format(na_rep='MISS', precision=3) # doctest: +SKIP
650                 0       1       2
651         0    MISS   1.000       A
652         1   2.000    MISS   3.000
653 
654         Using a ``formatter`` specification on consistent column dtypes
655 
656         >>> df.style.format('{:.2f}', na_rep='MISS', subset=[0,1])
Expected:
            0      1          2
    0    MISS   1.00          A
    1    2.00   MISS   3.000000
Got:
    <pandas.io.formats.style.Styler object at 0x7f77b214ea30>

/home/aneesh/Desktop/Side Projects/ops/pandas/pandas/io/formats/style_render.py:656: DocTestFailure
- generated xml file: /home/aneesh/Desktop/Side Projects/ops/pandas/test-data.xml -
============================= slowest 30 durations =============================
1.14s call     pandas/io/formats/style_render.py::pandas.io.formats.style_render.StylerRenderer.format

(2 durations < 0.005s hidden.  Use -vv to show these durations.)
=========================== short test summary info ============================
FAILED pandas/io/formats/style_render.py::pandas.io.formats.style_render.StylerRenderer.format
============================== 1 failed in 1.70s ===============================
```

---

## OBSERVATION

The mentioned test returns a Styler object which will render properly in (for instance, a notebook), but will fail the doctest as it will simply return the object. 

## SOLUTION

Add images of output rendered, in the doctest wherever required. (The approach is mentioned https://github.com/pandas-dev/pandas/issues/42674#issuecomment-887075744, for the documentation of [background_gradient()](https://pandas.pydata.org/docs/reference/api/pandas.io.formats.style.Styler.background_gradient.html)) or add a ```# doctest: +SKIP``` if necessary.
