Quick Documentation fixes for APT pkgrepo state

* the aptitude repo manager needs the name to be the complete
  entry from the sources.list file.  Documentation modified to
  make this clear (compared to say the YUM version)

* removed a superfluous reference to "deb" in the uri section
Merge pull request #3458 from morganfainberg/document-fixes-for-apt-pkrepo

Quick Documentation fixes for APT pkgrepo state