Updates link to the understanding-docker section

Signed-off-by: Spencer Smith <robertspencersmith@gmail.com>

docs/misc/index.md

@@ -73,7 +73,7 @@ Docker consists of:
 
 ## About this guide
 
-The [Understanding Docker section](introduction/understanding-docker.md) will help you:
+The [Understanding Docker section](/introduction/understanding-docker) will help you:
 
  - See how Docker works at a high level
  - Understand the architecture of Docker