Improve bin/config set errors for complex values

Summary:

• Fix missing space before "For example:".
• Fix instruction to run bin/config set value instead of bin/config set key value.
• Minor cleanup.

Test Plan: Tried to set files.image-mime-types, load-libraries.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D14080