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>

docs/using_helm.md

@@ -211,12 +211,12 @@ imageTag: 10.1.14-r3
 # mariadbDatabase:
 ```
 
-You can then override any of these settings in a YAML formatted file,
+You can then override any of these settings in a JSON or YAML formatted file,
 and then pass that file during installation.
 
 ```console
-$ echo '{mariadbUser: user0, mariadbDatabase: user0db}' > config.yaml
+$ echo '{mariadbUser: user0, mariadbDatabase: user0db}' > config.json
-$ helm install -f config.yaml stable/mariadb
+$ helm install -f config.json stable/mariadb
 ```
 
 The above will create a default MariaDB user with the name `user0`, and