Fix docs for configuring pygments themes

[philippjfr]

They are configured as part of the html_theme_options not in the html_context. See

pydata-sphinx-theme/src/pydata_sphinx_theme/__init__.py

Lines 622 to 625 in 3c7fa00

	# check the theme specified in the theme options
	theme_options = app.config["html_theme_options"]
	pygments_styles = list(get_all_styles())
	light_theme = theme_options.get("pygment_light_style", default_light_theme)

for reference.

[choldgraf]

Good catch 🙏