The link to image file of alphamat tutorial seems to be broken

[atinfinity]

System information (version)

• OpenCV => 4.5
• Operating System / Platform => ❔
• Compiler => ❔

Detailed description

The link to image file of alphamat tutorial seems to be broken.
https://docs.opencv.org/4.5.0/dd/d0e/tutorial_alphamat.html

Screenshot

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues,
  answers.opencv.org, Stack Overflow, etc and have not found solution
• I updated to latest OpenCV version and the issue is still there
• There is reproducer code and related data files: videos, images, onnx, etc

[alalek]

Fixed here: #2729