5.11 Uniforms

The answer may be quite simple if you remember.
The p-value must be between 0 and 1 and the uniform distribution must be between 0 and 1. The probability of the p-value is greater than x is (1-x). Similarly, the probability of a uniform distribution is greater than x is (1-x).
That should be all to say you need to show the two distributions are equal. Therefore, p-values are uniformly distributed.