docs: fix plugin docs typo

[Rohan5commit]

Summary

• Fixes a typo in the plugin documentation.
• Changes convertors to converters.

Related issue

N/A (trivial docs typo fix)

Guideline alignment

• https://github.com/httpie/cli/blob/master/CONTRIBUTING.md
• One focused docs change in one file.
• No behavior changes.

Validation

• Not run (documentation-only change).

[Copilot]

Pull request overview

Fixes a typo in the plugin documentation by correcting the term “convertors” to “converters” in the list of plugin types.

Changes:

• Replace “message convertors” with “message converters” in the plugin section of the docs.

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