R1:7425407c126b

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