Guides: renaming to active_record_validations_callbacks for consistency and updating links to reflect new path

Fixing some minor typos, grammar, accuracy, and spacing issues in Active Record Validations Callbacks guide

This reverts commit 51a9703b.

- "i.e." should be "e.g." in these cases, and include the comma
- Newline consistency before/after Ruby blocks
- Conform to the apparent guide standard of "Active Record" over "ActiveRecord"

Signed-off-by: NXavier Noria <fxn@hashref.com>