Merge pull request #21188 from UncleLLD:fix-markdown-error

fix issue 21187: markdown file: gray image does not have three dimensions

doc/py_tutorials/py_feature2d/py_feature_homography/py_feature_homography.markdown

@@ -78,7 +78,7 @@ if len(good)>MIN_MATCH_COUNT:
     M, mask = cv.findHomography(src_pts, dst_pts, cv.RANSAC,5.0)
     matchesMask = mask.ravel().tolist()
 
-    h,w,d = img1.shape
+    h,w = img1.shape
     pts = np.float32([ [0,0],[0,h-1],[w-1,h-1],[w-1,0] ]).reshape(-1,1,2)
     dst = cv.perspectiveTransform(pts,M)