[fix] make books/user.pdf

Error:

  Configuration error:
  There is a programmable error in your configuration file:
  ...
  NameError: name 'DOCS_URL' is not defined
  make: *** [utils/makefile.sphinx:156: books/user.latex] Fehler 2

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>