Fix 2.1 version in robots.txt

robots.txt

@@ -2,6 +2,6 @@ user-agent: *
 disallow: /*/3.2
 disallow: /*/3.1
 disallow: /*/3.0
-disallow: /*/2.0
+disallow: /*/2.1
 
 sitemap: https://docs.godotengine.org/sitemap.xml