fix thumbnail (#2386)

Author: hynky1999

_blog.yml

@@ -4714,7 +4714,7 @@
 - local: benczechmark
   title: "🇨🇿 BenCzechMark - Can your LLM Understand Czech?"
   author: mfajcik
-  thumbnail: /blog/assets/197_benczechmark/thumbnail.png
+  thumbnail: /blog/assets/187_benczechmark/thumbnail.png
   date: October 1, 2024
   tags:
     - nlp