revised BB vignette pdf.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/optimizer@884 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
minor edit.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/optimizer@885 edb9625f-4e0d-4859-8d74-9fd3b1da38cb