Gave a better name to the `--color` placeholder (#3226)

## Summary

The cli gives <COLOR> as value placeholder which is misleading. I
changed this to <COLOR_CHOICE> to make it obvious you don't supply a
color but a ColorChoice value.

## Test Plan

Compiled and verified --help text was correct

crates/uv/src/cli.rs

@@ -61,7 +61,8 @@ pub(crate) struct GlobalArgs {
         long,
         value_enum,
         default_value = "auto",
-        conflicts_with = "no_color"
+        conflicts_with = "no_color",
+        value_name = "COLOR_CHOICE"
     )]
     pub(crate) color: ColorChoice,