docs: point README imports at v4-core/src

[imitater-dou]

Summary

README local-usage Solidity imports still used v4-core/contracts/..., but on tip the tree (and published package) live under src/. contracts/ is gone (Contents API 404).

Repro

TIP=46c6834698c48bc4a463a86d8420f4eb1d7f3b75
curl -sI https://raw.githubusercontent.com/Uniswap/v4-core/$TIP/src/interfaces/IPoolManager.sol | head -1   # 200
curl -sI https://raw.githubusercontent.com/Uniswap/v4-core/$TIP/contracts/interfaces/IPoolManager.sol | head -1  # 404

Change

Update the two example imports in README.md from contracts/ → src/.

Notes

Open README rewrites #1020/#1016 do not fix these import paths.