added header: to k-means front matter

removed `image:` and replaced with `header:` to try to get a custom image for the post.

Author: mdcramer

_apple-2-blog/k-means.md

@@ -1,7 +1,9 @@
 ---
 title: "K-means by another means"
 excerpt: "Success! There is machine learning happening on my Apple ][+."
-image: /assets/images/apple2/k-means-convergence.jpg
+header:
+  overlay_image: /assets/images/apple2/k-means-convergence.jpg
+  og_image: /assets/images/apple2/k-means-convergence.jpg
 tags:
   - K-means
   - machine learning