Australian swimmer Ariarne Titmus dislodged American great Katie Ledecky as the world record holder in the women’s 400-metre freestyle with a stunning performance at the national championships in Adelaide on Sunday. The 21-year-old Titmus swam 3 minutes 56.40 seconds to beat Ledecky’s old record, set in winning gold at the 2016 Olympic Games in Rio de Janeiro, by 6 hundredths of a second. Titmus herself is a double Olympic champion from the delayed Tokyo 2020 Games held last year with one of her victories being in the 400m over Ledecky. She also won the 200m but the 25-year-old American remain…