Add initialisations to make clear what values actually have to be passed.

Moreover, in accordance with https://github.com/opencv/opencv/blob/383559c2285baaf3df8cf0088072d104451a30ce/modules/flann/include/opencv2/flann/defines.h#L68, I believe `FLANN_INDEX_KDTREE` was being initialised wrongly in the code examples, 1 should be correct, whereas 0 is `FLANN_INDEX_LINEAR`.