[fix] duckduckgo_definition issues reported by 'manage.sh test'

Fix this error while travis build::

  /home/travis/build/asciimoo/searx/searx/engines/duckduckgo_definitions.py:21:44: E225 missing whitespace around operator

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