Updated information in advocacy section

• Change the incorrect characters `` to ''
• Correct the URL that refers to the Handbook
• Change the URL of the *BSD conferentes to use https
• Update the number of FreeBSD and OpenBSD ports
• Some corrections with igor

Patch by: carlavilla@
Approved by: bcr@(mentor)
Differential Revision: https://reviews.freebsd.org/D23040