As if the corporate merger wave in the US was not moving fast enough already, The Christian Science Monitor has apparently joined two companies together on its own. A story about ''Strawberry Shortcake'' dolls that appeared on this page Dec. 29 stated that the manufacturer of these toys, American Greetings Company, was a division of General Mills. This is not the case. General Mills and American Greetings jointly launched the product, but American Greetings remains an independent corporation. The Monitor regrets the error.