Fix typo in the doc of multivariate monic orthogonal polynomials

[bowenszhu]

Original: $$w: \mathcal{W} = \mathcal{W}^{(1)} \otimes \mathcal{W}^{(2)} \mathcal{W}^{(1)} \cdots \otimes \mathcal{W}^{(p)} \rightarrow \mathbb{R}_{\geq 0}$$

Changed to: $$w: \mathcal{W} = \mathcal{W}^{(1)} \otimes \mathcal{W}^{(2)} \otimes \cdots \otimes \mathcal{W}^{(p)} \rightarrow \mathbb{R}_{\geq 0}$$