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

README.rst

@@ -75,7 +75,7 @@ Usage
 .. code:: bash
 
  # Use a pre-defined lint configuration
- yamllint -d relaxed file.yml
+ yamllint -d relaxed file.yaml
 
  # Use a custom lint configuration
  yamllint -c ~/myconfig file.yml
@@ -83,7 +83,7 @@ Usage
 .. code:: bash
 
  # Output a parsable format (for syntax checking in editors like Vim, emacs...)
- yamllint -f parsable file.yml
+ yamllint -f parsable file.yaml
 
 Configuration example
 ^^^^^^^^^^^^^^^^^^^^^