fix: figure caption counter

Before this commit, figure captions were always numbered as 1.

Author: andywiecko

style.css

@@ -191,7 +191,7 @@ body {
   margin: 0 auto;
   padding: 2rem 1.25rem;
 
-  counter-reset: theorem definition sidenote-counter;
+  counter-reset: theorem definition sidenote-counter figcaption;
 
   color: var(--body-color);
   background-color: var(--body-bg-color);