Typo in docstring of distanceTransform

[innerlee]

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

[alalek]

Thank you for contribution!

As a "bugfix" this patch should go into 3.4 branch first.
We will merge changes from 3.4 into master regularly (weekly/bi-weekly).

So, please:

• change "base" branch of this PR: master => 3.4 (use "Edit" button near PR title)
• rebase your commits from master onto 3.4 branch. For example:
  git rebase -i --onto upstream/3.4 upstream/master
  (check list of your commits, save and quit (Esc + "wq" + Enter)
  where upstream is configured by following this GitHub guide and fetched (git fetch upstream).
• push rebased commits into source branch of your fork (with --force option)

Note: no needs to re-open PR, apply changes "inplace".