Fixes a few things in the bsdl-gpl article:

inconsistency regarding AT&T at the beginning of the text, which later
becomes ATT: I choosed to always use AT&T;

style improvement: change "adds additional" into "adds more";

change "Addenda" into "Bibliographical References", since that is what is in the section;

fix typos.

Submitted by: salvadore@
Reviewed by: carlavilla@
Approved by: bcr@(mentor)
Differential Revision: https://reviews.freebsd.org/D23588