Readme file corrections

[farfun]

Correct README.md and src/funfake/headers/README.md to accurately reflect Python version, API usage, and internal data statistics.

The original README.md had inconsistencies:

1. Python version badge stated 3.6+ while pyproject.toml required 3.8+.
2. Headers(os='linux') example was incorrect; the actual parameter is os='lin'.
3. Browser User-Agent counts and ranges, as well as the Referer domain count, were outdated compared to the library's internal data.
   These changes ensure the documentation is consistent with the code and provides accurate information for users.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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