Align parachain example with Asset Hub terminology

[Devpen787]

Summary

Align the larger parachain-example with the Asset Hub terminology already used elsewhere in the repo.

This keeps the example behavior unchanged and only updates the stale Statemint naming in:

• examples/parachain-example/README.md
• examples/parachain-example/src/main.rs

What changed

• update the README narrative to refer directly to Asset Hub
• rename the generated module usage from statemint to asset_hub
• rename StatemintConfig to AssetHubConfig

What did not change

• no example logic changes
• no API changes
• no config changes
• no metadata file rename in this PR

I left statemint_metadata.scale as-is to keep the first pass narrow and avoid file-move churn.

Verification

• cargo check --manifest-path examples/parachain-example/Cargo.toml

[cla-bot-2021]

User @Devpen787, please sign the CLA here.