Fix active contour gallery example after change to rc coordinates

[sciunto]

Description

PR #4035 changed the active contour example to use rc coordinates rather than xy, but it did not update the matplotlib code, resulting in transposed contours. This change fixes that oversight.

Fixes #4253

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.
• Consider backporting the PR with @meeseeksdev backport to v0.14.x

[hmaarrfk]

These really need a before and after.

[jni]

@sciunto (and everyone) we haven't yet made a comment about it, but one of the issues @NelleV encountered while compiling the release notes is that many PRs don't provide sufficient information in the title and comments to make an informed decision about how to highlight that PR. This required excessive clicking through to fully understand a PR. Therefore, let's make an effort to write coherent summaries in the PR title and description.

btw, not to single you out at all, Nelle pointed out one of my PRs as a salient example. 😂 🤦‍♂

[sciunto]

Thanks for the comment, i'll try to improve that as well.

[jni]

Oh! And:

[jni]

@meeseeksdev backport to v0.16.x

[jni]

=P