Update using_helm.md (#4690)

* Update using_helm.md

In docs/using_helm.md, it says on line 214 that you can pass in a YAML formatted file, but the example commands following that sentence create JSON code but names the file with a .yml extension. For clarity, I propose saying that it will accept JSON or YAML but clarify in the code that for the example we're making a JSON file.
Signed-off-by: Pablo Canseco <p@blocanse.co>

* update using_helm.md to accurately say that helm install -f only takes a yaml-formatted file.
Signed-off-by: Pablo Canseco <p@blocanse.co>

* updated wording to reflect the fact that -f / --values only accepts YAML
Signed-off-by: Pablo Canseco <p@blocanse.co>

docs/using_helm.md

-# Using Helm
+# Using Helm
 
 This guide explains the basics of using Helm (and Tiller) to manage
 packages on your Kubernetes cluster. It assumes that you have already
@@ -215,7 +215,10 @@ You can then override any of these settings in a YAML formatted file,
 and then pass that file during installation.
 
 ```console
-$ echo '{mariadbUser: user0, mariadbDatabase: user0db}' > config.yaml
+$ cat << EOF > config.yaml
+mariadbUser: user0
+mariadbDatabase: user0db
+EOF
 $ helm install -f config.yaml stable/mariadb
 ```