Fix URL docstring typos

[Rohan5commit]

Summary

Fix two URL docstring typos in httpcore/_models.py.
This is a documentation-only change in a single file with no behavior impact.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Copilot]

Pull request overview

This PR corrects two spelling typos in the URL class docstring in httpcore/_models.py, improving the accuracy of the documentation without affecting runtime behavior.

Changes:

• Fixes “convienence” → “convenience” in the URL docstring.
• Fixes “explicitily” → “explicitly” in the URL docstring.

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