docs: fix simple typo, tranform -> transform (#234)

There is a small typo in demos/perspective_transform.py.

Should read `transform` rather than `tranform`.

Author: timgates42

demos/perspective_transform.py

@@ -20,7 +20,7 @@
 for (x, y) in pts:
     cv2.circle(clone, (x, y), 5, (0, 255, 0), -1)
 
-# apply the four point tranform to obtain a "birds eye view" of
+# apply the four point transform to obtain a "birds eye view" of
 # the notecard
 warped = perspective.four_point_transform(notecard, pts)