Fix option name in README.md.

[ezio-melotti]

Just a minor mistake in the documentation -- the path of the image defined in ogp_image is relative to ogp_site_url, not ogp_site_name:

sphinxext-opengraph/sphinxext/opengraph/__init__.py

Lines 142 to 148 in 9f1a8c8

	else: # ogp_image is set
	# ogp_image is defined as being relative to the site root.
	# This workaround is to keep that functionality from breaking.
	root = config["ogp_site_url"]
	image_url = urljoin(root, image_url_parsed.path)
	tags["og:image"] = image_url