Fix entity install-intro.docbook with wrong link

remove usage of entity &package; kde does only release single application tarballs (with a few exceptions like calligra, kdepim, kdewebdev)
&kde-ftp; and &FTP; are gone you can get source tarballs afaik only via download.kde.org

see also corresponding commmit in frameworks/doctools http://quickgit.kde.org/?p=kdoctools.git&a=commit&h=7957a1c24490e3d42f21983c0d237a3f208faabe
and RR https://git.reviewboard.kde.org/r/123850/