Update py_drawing_functions.rst

Hello,

cv2.line(), cv2.rectangle(), etc funtion returns None. In that case the img variable being assigned becomes None. As in python the 'img' parameter are passed by reference there is no need to assign the return of the function. 

The following is a working code in python. 


'''
Created on 23-Feb-2014

@author: niraj
'''
import numpy as np
import cv2

img = np.zeros((512,512,3), np.uint8)
cv2.line(img,(0,0),(511,511),(255,0,0),5)
cv2.rectangle(img,(384,0),(510,128),(0,255,0),3)
cv2.circle(img,(447,63), 63, (0,0,255), -1)
cv2.ellipse(img,(256,256),(100,50),0,0,180,255,-1) 
pts = np.array([[10,5],[20,30],[70,20],[50,10]], np.int32)
pts = pts.reshape((-1,1,2))
cv2.polylines(img,[pts],True,(0,255,255))
font = cv2.FONT_HERSHEY_SIMPLEX
cv2.putText(img,'OpenCV',(10,500), font, 4,(255,255,255),2)
    
cv2.imshow('image', img)

cv2.waitKey(0)
cv2.destroyAllWindows()