Clarify label encoding comment in char_rnn_classification_tutorial

[patrocinio]

The previous comment 'Cache the tensor representation of the labels' was misleading, especially for beginners. The code actually creates a numerical representation by storing unique labels in alphabetical order and converting each label to its corresponding index as a tensor.

Updated the comment to clearly explain what's happening: creating numerical representations rather than caching.

Fixes #3646

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• [ ] The issue that is being fixed is referred in the description (see above "Fixes #ISSUE_NUMBER")
• [ ] Only one issue is addressed in this pull request
• [ ] Labels from the issue that this PR is fixing are added to this pull request
• [ ] No unnecessary issues are included into this pull request.