Fix breadcrumb label

Author: cyderize

src/pages/challenge/2021/index.mdx

@@ -8,7 +8,7 @@ import Head from 'next/head'
 
 <Breadcrumbs
   className="not-prose"
-  items={[['MiniZinc Challenge', '/challenge'], 'Challenge 2023']}
+  items={[['MiniZinc Challenge', '/challenge'], 'Challenge 2021']}
 />
 
 ### The Challenge