Accessibility Principles: Clarify "enlarged to custom text sizes"

[remibetin]

Re-creates @MakotoUeki's https://github.com/w3c/wai-accessibility-principles/pull/29

Preview: https://deploy-preview-839--wai-website.netlify.app/fundamentals/accessibility-principles/#perceivable

Initial description

It can be read as if users can enlarge text which is the value of alt attribute. It is confusing.

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