Merge pull request #13104 from alalek:docs_fix_bib_Kroeger2016

docs: fix Kroeger2016 bib reference

move from opencv_contrib

modules/video/doc/video.bib

+@inproceedings{Kroeger2016,
+  author={Till Kroeger and Radu Timofte and Dengxin Dai and Luc Van Gool},
+  title={Fast Optical Flow using Dense Inverse Search},
+  booktitle={Proceedings of the European Conference on Computer Vision ({ECCV})},
+  year = {2016}
+}