Cher is bringing readers inside the exact moment her marriage to Sonny Bono officially ended. In an excerpt from the upcoming Cher: The Memoir, Part One, which will be released on November 19, the pop icon, 78, wrote about how a sudden kiss with a young guitarist was the final straw in the demise of her dwindling romance with her former husband. RadarOnline.com can reveal that Cher heard through a friend that a man named Bill, who joined the couple’s band, had a “crush” on her. Although the Mermaids star originally brushed off the idea because of her union with Sonny, she began to entertain th…