Fix atf-sh(3) manpage issues

• Fix spelling errors (specifiying -> specifying) [1]
• Escape '.' at start of lines using & to tell the roff processor that the line isn't meant to be treated as a command [2].

Bump .Dd for the change

MFC after: 1 week
Reported by: igor [1], manlint [2]
Sponsored by: Dell EMC Isilon