Fix flag name to retrieve if runtime plugins are configurable (#2885)

## Motivation and Context
Follow-up on #2864. It got it 99% right, it's just that the name of a
flag for retrieving its value was incorrect.

## Testing
Generated an AWS SDK and confirmed that setters on `Config` for runtime
plugins are marked as `pub(crate)` correctly.

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Co-authored-by: ysaito1001 <awsaito@amazon.com>