linear forward example

[lheagy]

Update the notation in the forward mapping to use m1, m2 rather than f, g (which are easily confused with kernels)

$\mathcal{F}[\alpha f + \beta g]$ --> $\mathcal{F}[\alpha m_1 + \beta m_2]$

In the page: https://geoscixyz.github.io/inversion/forward-mapping#linear-and-nonlinear-mappings