Update docstring for dilation, which is now nD.

[mkcor]

Description

Follows from #5262 (comment).

I'm not sure x is a great (conventional?) notation for a pixel's coordinates (no matter how many the dimensions are)... @grlee77 thoughts?

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
• Descriptive commit messages (see below)

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.

[rfezzani]

Thank you @mkcor 😉