docs: clarify file-like object comment

[Rohan5commit]

Summary

• Replace the ambiguous fo shorthand with file-like object in the body preparation comment.

Related issue

• N/A

Guideline alignment

• Read the contribution guide where present and kept this to one focused text-only file change.
• No behavior, test, fixture, changelog, or generated-file changes.

Validation

• git diff --check
• Not run: broader tests are unnecessary for this text-only change.

AI assistance

• Prepared with Codex and reviewed before submission.

[Copilot]

Pull request overview

This PR clarifies an inline comment in PreparedRequest.prepare_body by replacing the ambiguous shorthand fo with the explicit phrase “file-like object,” improving readability without changing behavior.

Changes:

• Update the body-preparation comment to explicitly describe “file-like object” rather than fo.

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