The main bases on which we can make this argument have to do with the Vietnam War, with China, and with the Cold War era of détente.
With regard to Vietnam, President Nixon can be credited with ending the war. The war had been dragging on and could have lasted a lot longer had the US been inclined to keep fighting. However, Nixon did not choose this path. Instead, he chose to Vietnamize the war and then to end it completely. This can be seen as a major contribution to world peace.
Nixon is also famous for having gone to China and having opened diplomatic relations with that country. Ever since the end of the Chinese Civil War in 1949, the US had not had any diplomatic relations with Mainland China, which was ruled by the communists. Nixon chose to end that by working hard to establish contact with China. His efforts culminated in his trip to China in 1973.
Finally, Nixon is associated with the period of détente with the Soviet Union. It was under Nixon, for example, that the US and the Soviet Union made the SALT I pact. This was the first major nuclear arms limitation treaty between the two superpowers.
In these ways, Nixon can be credited with actions that helped to promote world peace.