Change the og:image on /learn back to the default

[joshlory]

Default is default-og-image.png set here:

code-dot-org/pegasus/router.rb

Lines 488 to 494 in 69c4770

	unless metadata['og:image']
	if request.site != 'csedweek.org'
	metadata['og:image'] = CDO.code_org_url('/images/default-og-image.png', 'https:')
	metadata['og:image:width'] = 1220
	metadata['og:image:height'] = 640
	end
	end