More anti-Semitic graffiti appeared in Rome Tuesday not far from the Great Synagogue of Rome after graffiti was daubed on a synagogue in the Monteverde district Monday and a plaque commemorating a two-year-old boy killed by Palestinian militants at the Synagogue in October 1982 was defaced.     "Zionist Jews, cancer of the world," was written along the Lungotevere degli Anguillara, not far from Rome's former ghetto and Synagogue, on the opposite side of the Tiber, near the entrance to Tiber