Fix batch email configuration docs

[ccmaymay]

Fix batch_email section name, email_interval parameter name in configuration docs.

I don't think there's much to test; I have been trying to get email notifications working in an application, and they only started working as expected once I discovered these documentation errors and corrected the section/parameter names in my code.

Closes #3293, #3294.

[dlstadther]

The docs referenced the config section and name that would have been used by the original configuration system (which was deprecated but not fully cleaned up; I had actually cleaned up this deprecation in the scheduler). You can see here that the email_interval had a config_path which referenced the section and name from the docs. However, in 2015 the original Luigi author updated the config section parsing to default to the config class name and its property names.

Once the CI completes (i updated this branch with master to resolve an issue with CI), I'll merge.