Update index.md

Author: ChrisRackauckas

docs/src/index.md

@@ -51,7 +51,7 @@ end
 This setup defines a type structure which is both a tree and an array. A picture of a possible
 version is the following:
 
-<img src="https://user-images.githubusercontent.com/1814174/27211626-79fe1b9a-520f-11e7-87f1-1cb33da91609.PNG">
+![](https://user-images.githubusercontent.com/1814174/27211626-79fe1b9a-520f-11e7-87f1-1cb33da91609.PNG)
 
 Let's build a version of this. Using the constructors we can directly construct leaf types: