* 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>