Fixed the document of alphamat tutorial

[atinfinity]

I resolved #2742.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

force_builders_only=docs

[alalek]

Please take a look on Doxygen demo page for this PR.

Avoid using of relative paths, please use alphamat/samples/... prefix (<opencv_contrib>/modules is added to a search list of base image paths).

Also please revert table layout for this, it is not handled properly by Doxygen.

[alalek]

Thank you for contribution!
Fixed here: #2729