docs: There are sentences in the contributors file that are not punctuated

I found that there are sentences in the contributors file that are not punctuated and I am addressing it.

Author: cubxxw

contributors/devel/README.md

@@ -9,7 +9,7 @@ Guide](http://kubernetes.io/docs/admin/).
 ## The process of developing and contributing code to the Kubernetes project
 
 * **Contributor Guide**
-  ([Please start here](/contributors/guide/README.md)) to learn about how to contribute to Kubernetes
+  ([Please start here](/contributors/guide/README.md)) to learn about how to contribute to Kubernetes.
 
 * **GitHub Issues** ([/contributors/guide/issue-triage.md](/contributors/guide/issue-triage.md)): How incoming issues are triaged.