Fix invalid order field names in help text and README

The API accepts `volume` and `liquidity` as order fields, not
`volume_num` and `liquidity_num` (those are response fields).
Using the documented examples produced a 422 validation error.

Author: rursache

README.md

@@ -128,7 +128,7 @@ Errors follow the same pattern — table mode prints `Error: ...` to stderr, JSO
 ```bash
 # List markets with filters
 polymarket markets list --limit 10
-polymarket markets list --active true --order volume_num
+polymarket markets list --active true --order volume
 polymarket markets list --closed false --limit 50 --offset 25
 
 # Get a single market by ID or slug

src/commands/markets.rs

@@ -42,7 +42,7 @@ pub enum MarketsCommand {
         #[arg(long)]
         offset: Option<i32>,
 
-        /// Sort field (e.g. `volume_num`, `liquidity_num`)
+        /// Sort field (e.g. `volume`, `liquidity`)
         #[arg(long)]
         order: Option<String>,