Not quite. There are 10 rivers that originate in Tibet, six of which are considered major rivers. They provide water to about 20% of the world’s population, but they do not constitute 25% of all the rivers in the world.

The Tibetan Plateau is the largest store of freshwater outside of the two polar regions and so is sometimes called the “Third Pole”. I can’t find any data on what percentage of the world’s freshwater is contained in the Tibetan Plateau, however.