Fix ignore folders documentation
Fix ignore folders documentation

It appears that we are documenting that to ignore indexing folders in elasticsearch, we have to use the `fs.ignore_folders` setting.
But this setting does not exist and is actually `index_folders` with the opposite meaning.

This commit fixes the documentation.

Closes #327.

dadoonet/fscrawler,dadoonet/fscrawler,dadoonet/fsriver,dadoonet/fscrawler,dadoonet/fsriver,dadoonet/fsriver