Fix doc visibility issue with `HickoryDnsResolverBuilder` (#4296)

## Motivation and Context
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Noticed that the
[HickoryDnsResolverBuilder](https://docs.rs/aws-smithy-dns/0.1.0/aws_smithy_dns/struct.HickoryDnsResolver.html#method.builder)
was not visible in the `aws-smithy-dns` docs, this fixes that.

## Testing
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to -->
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Ran `cargo doc --all-features` locally and was able to see the builder
type.

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