Singular Values & the SVD · Superchallenge problem

Let $P = \begin{pmatrix}1 & 1 & 1\\1 & 2 & 3\\1 & 3 & 6\end{pmatrix}$ (the $3\times 3$ symmetric Pascal matrix) and let $P_1$ be its best rank-one approximation. Find $\|P - P_1\|_F$. Give your answer in simplest radical form.