docs: fix asciinema cast

Author: boneskull

site/src/content/docs/getting-started/index.mdx

@@ -184,7 +184,7 @@ modestbench "tests/**/*.bench.ts"
 modestbench provides clean, colorized output in interactive terminals:
 
 <AsciinemaPlayer
-  src="/modestbench/modestbench.cast"
+  src="/modestbench.cast"
   autoPlay={true}
   loop={false}
   speed={1.0}