An 11-year-old boy in Florida was diagnosed with bone cancer five years ago, and had his right leg amputated. But that hasn’t stopped him from competing in kids’ triathlons. On Sunday, he did a 150-yard swim and a three-mile bike ride . . . but during the one-mile run, one of the screws on his prosthetic leg came loose and he fell down. Luckily, a Marine volunteering at the race picked him up and carried him across the finish line.