Replace hardcoded 'ImageMagick' with generic 'variant processor' in Guides

[Zhong-z]

Motivation / Background

This Pull Request has been created because the guide of configuring ActiveStorage variable_content_types is hardcoded with 'ImageMagick' when the processor is configurable.

Detail

This Pull Request changes the hardcoded 'ImageMagick' to say 'variant processor'

Additional information

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• There are no typos in commit messages and comments.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Feature branch is up-to-date with main (if not - rebase it).
• Pull request only contains one commit for bug fixes and small features. If it's a larger feature, multiple commits are permitted but must be descriptive.
• Tests are added if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.
• PR is not in a draft state.
• CI is passing.