Fix issues raised in the pull request

@ametaireau suggested few changes in his
[review](https://github.com/getpelican/pelican-plugins/pull/7).

Summary of his suggestions is as follows

1. Remove docstring because it is already present in the ReadMe file
1. No need to check for `keys` in  `gen.settings.keys()`
1. `goodreads_instance` should be renamed

This commit fixes all three issues.