rIn the new comedy “The Ringer,” Johnny Knoxville’s character learns to love the Special Olympics, yelling at his uncle, saying he never wants to hear him say the words “retarded” or “ ’tard” ever again. He might succeed on film.But could he persuade Bostonians, who say “retahded” in casual conversation without hesitation, to change their slang? “People use it in a disparaging way,” said Leo Sarkissian, executive director of The ARC of Massachusetts. “We took it out of our name,” Sarkissian said. The group goes by ARC specifically to avoid saying the word (although it’s part of the acronym Association...