Fix the example codes in the contributors guides

[seisman]

Preview: https://pygmt-dev--2929.org.readthedocs.build/en/2929/contributing.html#contributing-code

[yvonnefroehlich]

To be consistent, we also may want to update:

pygmt/doc/contributing.md

Lines 580 to 587 in bb82345

	```python
	@pytest.mark.mpl_image_compare
	def test_my_plotting_case():
	"Test that my plotting method works"
	fig = Figure()
	fig.basemap(region=[0, 360, -90, 90], projection="W15c", frame=True)
	return fig
	```

to

@pytest.mark.mpl_image_compare 
def test_my_plotting_case(): 
    """
    Test that my plotting method works.
    """ 
    fig = Figure() 
    fig.basemap(region=[0, 360, -90, 90], projection="W15c", frame=True) 
    return fig 

[yvonnefroehlich]

And maybe it makes sense to use code highlighting (```python .... ```) for the following code block:

pygmt/doc/contributing.md

Lines 334 to 340 in bb82345

	Example
	-------
	>>> import pygmt
	>>> # Comment describing what is happening
	>>> Code example

[seisman]

Thanks. They all make sense. Now fixed in 98b4331.

[yvonnefroehlich]

Should be fine now 🙂.