Restore Doxyfile version

Revert mistaken change from https://github.com/urho3d/Urho3D/commit/ef1dfe3417fed9e94a492e819df5dcb34faa6514

Docs/Doxyfile.in

@@ -1,4 +1,4 @@
-# Doxyfile 1.8.17
+# Doxyfile 1.8.13
 
 # This file describes the settings to be used by the documentation system
 # doxygen (www.doxygen.org) for a project.