docs: Import 'yamllint.config' rather than 'yamllint'

Since yamllint version 0.5.2 (in 2016, just after yamllint was created)
this example in documentation doesn't work. Indeed, commit dbbecb5
(which aimed to solve another problem) removed the ability to import
yamllint submodules directly.

This commit makes it clearer inside documentation.

Fixes #698

docs/development.rst

@@ -8,7 +8,8 @@ Basic example of running the linter from Python:
 
 .. code-block:: python
 
-   import yamllint
+   import yamllint.config
+   import yamllint.linter
 
    yaml_config = yamllint.config.YamlLintConfig("extends: default")
    for p in yamllint.linter.run(open("example.yaml", "r"), yaml_config):