Fixed broken latex in _posts/2015-10-20-math.md (#2219)

Latex math is currently broken in
[_posts/2015-10-20-math.md](https://alshedivat.github.io/al-folio/blog/2015/math/)
. Fixed it by replacing starts with underscore

---------

Co-authored-by: George <31376482+george-gca@users.noreply.github.com>

.prettierignore

@@ -3,3 +3,4 @@
 assets/css/main.scss
 assets/plotly/demo.html
 lighthouse_results/**
+_posts/2015-10-20-math.md

_posts/2015-10-20-math.md

@@ -21,7 +21,7 @@ MathJax will automatically number equations:
 
 \begin{equation}
 \label{eq:cauchy-schwarz}
-\left( \sum*{k=1}^n a_k b_k \right)^2 \leq \left( \sum*{k=1}^n a*k^2 \right) \left( \sum*{k=1}^n b_k^2 \right)
+\left( \sum_{k=1}^n a_k b_k \right)^2 \leq \left( \sum_{k=1}^n a_k^2 \right) \left( \sum_{k=1}^n b_k^2 \right)
 \end{equation}
 
 and by adding `\label{...}` inside the equation environment, we can now refer to the equation using `\eqref`.