Lev Manovich Typo Fix

This change fixes a typo in Lev Manovich's essay -- the date that
currently reads 1993 should be 1933, as noted in the wikipedia article
cited in the essay. Thanks to Kathryn Bloom (@Bloomjerseygirl) for
pointing out the error -
https://twitter.com/Bloomjerseygirl/status/308706774687551488