First equation in documentation is broken

[wsshin]

The very first equation in the documentation is broken. Here is the screenshot:

I verify the issue occurs on multiple browsers (Safari and Chrome on macOS; Safari on iOS).

Not sure why this is happening, though. The LaTeX expression is at Line 13 of docs/src/index.md:

$$f(x) \approx \sum_{k=1}^n c_k \psi_k(x)$$

but a similar equation at Line 23 of docs/src/usage/spaces.md does not break:

$$f(x) = \sum_{k=0}^\infty f_k T_k(x)$$