libtiff homepage is moved to http://www.simplesystems.org/libtiff/ around 2004.
See https://en.wikipedia.org/wiki/LibTIFF

However, some files include old URLs, they should be update.
This fix contains only changing source code comment, not chaning any behavious.

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 another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the 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