[action] cocoapods: replace clean -> clean_install in example

[WezSieTato]

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

clean in cocoapods actions is marked as deprecated

Description

From available_options:

         FastlaneCore::ConfigItem.new(key: :clean,
                                       env_name: "FL_COCOAPODS_CLEAN",
                                       description: "(Option renamed as clean_install) Remove SCM directories",
                                       deprecated: true,
                                       is_string: false,
                                       default_value: true)

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[WezSieTato]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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[fastlane-bot]

Hey @WezSieTato 👋

Thank you for your contribution to fastlane and congrats on getting this pull request merged 🎉
The code change now lives in the master branch, however it wasn't released to RubyGems yet.
We usually ship about once a week, and your PR will be included in the next one.

Please let us know if this change requires an immediate release by adding a comment here 👍
We'll notify you once we shipped a new release with your changes 🚀

[fastlane-bot]

Congratulations! 🎉 This was released as part of fastlane 2.147.0 🚀