Ruth Westheimer, champion of good sex, good contraception and good information, has a bad error in her sex guide for teen-agers that has prompted a recall of all 115,000 copies. In a chapter on avoiding pregnancy, the sex therapist and radio-TV lecturer, known to her admirers as "Dr. Ruth," mistakenly tells girls that they are least likely to get pregnant at times when they really are most fertile. "I'm very upset that that happened," Westheimer said. "I immediately said on my television show and the radio that there is a typo, that we are going to recall all the books." Her publisher will replace the white-covered $3.50 paperback with a red-covered corrected version, said Barbara Uva, speaking for Warner Books.