A few example paths are not recognized by dartdoc for missing the initial slash.

This is the error log thrown by dartdoc before the PR:

  warning: example file not found: example/lib/magnifier/cupertino_text_magnifier.0.dart
    from magnifier.CupertinoTextMagnifier: (file:///Users/tongmu/dev/packages/packages/cupertino_ui/lib/src/magnifier.dart:39:7)
  warning: example file not found: example/lib/magnifier/cupertino_magnifier.0.dart
    from magnifier.CupertinoMagnifier: (file:///Users/tongmu/dev/packages/packages/cupertino_ui/lib/src/magnifier.dart:251:7)
  warning: example file not found: example/lib/magnifier/text_magnifier.0.dart
    from text_field.CupertinoTextField.magnifierConfiguration: (file:///Users/tongmu/dev/packages/packages/cupertino_ui/lib/src/text_field.dart:850:37)

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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