fix: possessive error in Markdown cell

[CharlesCNorton]

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This PR addresses and corrects the possessive error in a Markdown cell. Correct "it's" to "its" in the sentence "GPT-3 didn't see during its pre-training" to ensure proper grammar. "It's" is a contraction for "it is" or "it has," whereas "its" is the possessive form of "it."

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Checklist:

• [ ] I have added a new entry in registry.yaml (and, optionally, in authors.yaml) so that my content renders on the cookbook website. (Not applicable)
• [x] I have conducted a self-review of my content based on the contribution guidelines:
  • [x] Relevance: This content is related to building with OpenAI technologies and is useful to others.
  • [x] Uniqueness: I have searched for related examples in the OpenAI Cookbook and verified that my content offers new insights or unique information compared to existing documentation.
  • [x] Spelling and Grammar: I have checked for spelling or grammatical mistakes.
  • [x] Clarity: I have done a final read-through and verified that my submission is well-organized and easy to understand.
  • [x] Correctness: The information I include is correct and all of my code executes successfully.
  • [x] Completeness: I have explained everything fully, including all necessary references and citations.