replaced "Vorbereiten zum" by "Demnächst" in Germa

When listening to Osmand's speech while navigating, I often hear from German TTS:
"Vorbereiten zum in 100 Meter Rechts abbiegen" what sounds really clumsy.

"Demnächst in 100 Meter abbiegen" or "Demnächst rechts abbiegen" sounds really better.
Merge pull request #193 from stephan75/patch-3

replaced "Vorbereiten zum" by "Demnächst" in German