Update GenerationConfig.md to fix typo

[odevop]

Fixed typo in Frequency Penalty description by changing "dificult" to "difficult".

Description of the change

Fixed an unfortunate misspelling of the word "difficult". The current documentation spells the word as "dificult" (one 'f') when the correct spelling is "difficult" (two 'f').

Motivation

Implementing this change will improve the professionalism of the documentation and prevent readers from being caught off guard/distracted by the misspelling of a word.

Type of change

Documentation

Checklist

• [ x] I have performed a self-review of my code.
• [ x] I have added detailed comments to my code where applicable.
• [ x] I have verified that my change does not break existing code.
• [ x] My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• [ x] I am familiar with the Google Style Guide for the language I have coded in.
• [ x] I have read through the Contributing Guide and signed the Contributor License Agreement.

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