Use '.yaml' extension as default, not '.yml'

As someone said [1] on the internet:

    Say ".yaml" not ".yml".
    This is not MS-DOS, and YML is a Yahoo XML dialect.

Similarly, we use '.json', not '.jsn'.

[1]: https://github.com/ceph/s3-tests/commit/e17c56a

docs/quickstart.rst

@@ -81,7 +81,7 @@ option:
 
 .. code:: bash
 
- yamllint -c ~/myconfig file.yml
+ yamllint -c ~/myconfig file.yaml
 
 .. note::