Simplify numpy function call in object detection tutorial

[ashok-arjun]

In the second code block in the tutorial, the line pos = np.where(masks[i]) has been used to get the indices of the non zero points in the image. But numpy documentation for np.where() advises to use np.nonzero() when there is only one argument for np.where(), and it also makes the code more readable.

[moDallel]

Hello !
We are a group of students in second year in engineering school. We are currently interested in resolving this issue as a school project. Please let me know, if we can have your permision to contribute on this issue.

[JoseLuisC99]

/assigntome

[JoseLuisC99]

I consider that #2372 resolves this issue.

[sekyondaMeta]

Closing this issue as it is resolved by PR #2372