Fix invalid order field names in help text and README

[rursache]

Summary

• Fixes --order volume_num → --order volume in README example and CLI help text
• The API rejects volume_num and liquidity_num as order parameters with a 422 error; the valid values are volume and liquidity

Root cause

volume_num and liquidity_num are field names on the Market response object (used for display), but the API /markets endpoint expects volume and liquidity as order query parameter values. The README and CLI help text were using the response field names instead of the query parameter names.

Test plan

• cargo test — all tests pass
• polymarket markets list --active true --order volume returns results
• polymarket markets list --active true --order volume_num returns 422 (before fix, this was the documented usage)

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Note

Low Risk
Documentation/help-text only; no runtime logic changes beyond the displayed usage guidance.

Overview
Fixes markets list --order documentation to use API-valid sort fields: updates the README example and the CLI flag help text from volume_num/liquidity_num to volume/liquidity to avoid 422 errors from the /markets endpoint.

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