Update README.md

add `of` in first sentence

Author: perryao

docs/book/README.md

@@ -6,7 +6,7 @@
 
 #### Users of Kubernetes
 
-Users of Kubernetes will develop a deeper understanding Kubernetes through learning
+Users of Kubernetes will develop a deeper understanding of Kubernetes through learning
 the fundamental concepts behind how APIs are designed and implemented.  This book
 will teach readers how to develop their own Kubernetes APIs and the
 principals from which the core Kubernetes APIs are designed.