Swapped old import convention in examples for new one

[AlexLouk]

Description

This PR changes some of the examples old import convention for the new one (import skimage as ski) Resolves #7454 partly

Checklist

• A descriptive but concise pull request title
• Docstrings for all functions
• Unit tests
• A gallery example in ./doc/examples for new features
• Contribution guide is followed

Release note

For maintainers and optionally contributors, please refer to the instructions on how to document this PR for the release notes.

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[AlexLouk]

@mkcor do I need to do something to fix the failed checks ?

[mkcor]

@AlexLouk seeing no failed checks at the moment :wink: