ENH: lower momentum subscripts in LaTeX rendering

[redeboer]

Improvement for #123 and ComPWA/polarimetry#358. It's subtle, but maybe it helps 😅

Old

$$\begin{array}{rcl} \mathcal{R}^\mathrm{Flatté}\left(s\right) &=& \frac{1}{m^{2} - i m \left(\frac{\Gamma_{1} m p_{m_{1},m_{2}}\left(s\right)}{\sqrt{s} p_{m_{\pi},m_{\Sigma}}\left(m^{2}\right)} + \frac{\Gamma_{2} m p_{m_{\pi},m_{\Sigma}}\left(s\right)}{\sqrt{s} p_{m_{\pi},m_{\Sigma}}\left(m^{2}\right)}\right) - s} \\\ \end{array}$$

New

$$\begin{array}{rcl} \mathcal{R}^\mathrm{Flatté}\left(s\right) &=& \frac{1}{m^{2} - i m \left(\frac{\Gamma_{1} m p_{_{m_{1},m_{2}}}\left(s\right)}{\sqrt{s} p_{_{m_{\pi},m_{\Sigma}}}\left(m^{2}\right)} + \frac{\Gamma_{2} m p_{_{m_{\pi},m_{\Sigma}}}\left(s\right)}{\sqrt{s} p_{_{m_{\pi},m_{\Sigma}}}\left(m^{2}\right)}\right) - s} \\\ \end{array}$$