Fix incorrect documentation specification

[Anon-im]

Motivation

The constructor specifies that the url is set “forever”, but this is not reflected in the variable comment:

Change Summary

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Merge Checklist

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Additional Context

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PR-Codex overview

This PR updates the documentation for the url variable in the src/FnameResolver.sol file to clarify that it is set once during deployment and cannot be changed.

Detailed summary

• Updated the comment for the url variable to specify that it is set once during deployment and cannot be changed.

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