Fringes become closer together.
Work Step by Step
This is shown in Figure 29.14. The equation at the bottom of page 550 explains this. $$\lambda = d sin \theta$$ Assume the wavelength, the left-hand side of the equation, is fixed. Therefore, the product of the 2 terms on the right-hand side must be constant too. Suppose the distance between the slits, d, is increased. In that case, we see a more tightly-packed pattern, i.e., $\theta$ decreases, because sin $\theta$ must decrease.