Label the appendix simply as "Appendix: The rexp.proto schema
descriptor" and refer to it in line as "the appendix" instead of
adding a section "Appendices" with only a single appendix "A" in it.

Suggested by Dirk.

Also, add missing \proglang{} in the intro.


git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rprotobuf@813 edb9625f-4e0d-4859-8d74-9fd3b1da38cb