advocacy/myths.adoc: correct title and subheadings

The title of https://www.freebsd.org/advocacy/myths/ is identical to
the title of https://www.freebsd.org/advocacy/

• correct the title
• rationalise subheadings.

Approved by: gjb (mentor), carlavilla (mentor)
Differential revision: https://reviews.freebsd.org/D37244