Update deprecated URL for Helm's chart format in kubernetes.io

Currently, the URL to **Chart format** in **kubernetes.io** is out of date.
This commit aims to update this URL to the working one in Helm's github that
user are able to access.

Signed-off-by: Kim Bao Long <longkb@vn.fujitsu.com>

_proto/hapi/chart/metadata.proto

@@ -32,7 +32,7 @@ message Maintainer {
 
 //	Metadata for a Chart file. This models the structure of a Chart.yaml file.
 //
-// 	Spec: https://k8s.io/helm/blob/master/docs/design/chart_format.md#the-chart-file
+// 	Spec: https://helm.sh/docs/developing_charts/#the-chart-yaml-file
 message Metadata {
 	enum Engine {
 		UNKNOWN = 0;