fix(config): align unset help wording

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53171.

Summary

• Change config unset help text from "configuration value" to "configuration key" so it matches the existing success output and the command argument name.

Validation

• cargo +1.90.0 fmt --package cortex-cli --check
• git diff --check

Targeted cargo +1.90.0 test -p cortex-cli test_config_unset_subcommand -- --nocapture is blocked in this Windows/MSVC environment before crate tests run by the existing aws-lc-sys C build failure.

[greptile-apps]

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