P/B is isomorphic to the projective line $\mathbb{P}^1$

Question

Suppose that $P \subset G$ is a parabolic subgroup containing a Borel subgroup $B$. Moreover, let $P$ be a minimal parabolic subgroup properly containing B, i.e., one corresponding to a single root $\alpha$. Why is $P/B$ isomorphic to the projective line $\mathbb{P}^1$?