docs: fix README search examples to match current CLI and SDK

[priotts]

Fixes the README search examples, which use a syntax the current CLI/SDK doesn't accept.

• Quick Start: opensea search collections "cool cats" -> opensea search "cool cats" --types collection
• SDK snippet: client.search.collections(...) -> client.search.query(..., { assetTypes: ["collection"] })

Both now match docs/cli-reference.md, docs/examples.md and docs/sdk.md. Verified against the built package: the old snippet throws client.search.collections is not a function, the new one returns results.

Related to #53

[ryanio]

Thanks for this and for the clear write-up in #53. We've recreated the README fix in our internal monorepo in ProjectOpenSea/opensea-devtools#789 (now merged) with you credited, and it will reach this repo with the next CLI release.

This repo is a read-only mirror, so we can't merge PRs directly, but we review every one.