blog: fix-blog-gsoc-image (#234)

Author: xmnlab

pages/blog/three-years-with-google-summer-of-code-what-ive-learned/header.png

@@ -11,7 +11,7 @@ description: |
   playbook is simple—balance mentors, set explicit contribution rules, meet 
   regularly; and for contributors: communicate publicly and ship small, 
   tested PRs.
-thumbnail: "/header.svg"
+thumbnail: "/header.png"
 template: "blog-post.html"
 ---