added teaser to k-means front matter

added `teaser` to k-means front matter to see if that would trigger the custom og image without adding an overlay to the top of the post.

Author: mdcramer

_apple-2-blog/k-means.md

@@ -2,6 +2,7 @@
 title: "K-means by another means"
 excerpt: "Success! There is machine learning happening on my Apple ][+."
 header:
+  teaser: /assets/images/apple2/k-means-convergence.jpg
   og_image: /assets/images/apple2/k-means-convergence.jpg
 tags:
   - K-means