MAINT: use internal function for noise + clarify code in canny example

[sciunto]

Description

I propose to update the canny example to make it more "modern": variable name, noise function...

Here is the new image

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.

[jni]

great! 😊

[hmaarrfk]

nice.