Fix `FLANN_INDEX_KDTREE`, add `FLANN_INDEX_LSH` initialisation

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

Moreover, in accordance with 383559c228/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`.

doc/py_tutorials/py_feature2d/py_matcher/py_matcher.markdown

@@ -148,11 +148,13 @@ its related parameters etc. First one is IndexParams. For various algorithms, th
 passed is explained in FLANN docs. As a summary, for algorithms like SIFT, SURF etc. you can pass
 following:
 @code{.py}
+FLANN_INDEX_KDTREE = 1
 index_params = dict(algorithm = FLANN_INDEX_KDTREE, trees = 5)
 @endcode
 While using ORB, you can pass the following. The commented values are recommended as per the docs,
 but it didn't provide required results in some cases. Other values worked fine.:
 @code{.py}
+FLANN_INDEX_LSH = 6
 index_params= dict(algorithm = FLANN_INDEX_LSH,
                    table_number = 6, # 12
                    key_size = 12,     # 20
@@ -179,7 +181,7 @@ kp1, des1 = sift.detectAndCompute(img1,None)
 kp2, des2 = sift.detectAndCompute(img2,None)
 
 # FLANN parameters
-FLANN_INDEX_KDTREE = 0
+FLANN_INDEX_KDTREE = 1
 index_params = dict(algorithm = FLANN_INDEX_KDTREE, trees = 5)
 search_params = dict(checks=50)   # or pass empty dictionary