docs(web-client): fix broken API reference link in README

[midasbal]

Small one, spotted while reading through the web-client README. The "API Reference" link in the Usage section points at ../../docs/typedoc/web-client/README.md, which doesn't actually resolve anywhere a reader would land:

• it's gitignored generated typedoc output, so it's not there on a fresh clone
• it's not in the package's files array, so it doesn't ship in the npm tarball (and the README does, so npm readers hit a dead link)
• web-sdk isn't ingested into docs.miden.xyz, so it's not on the site either

Looks like it's been like this since the initial port in #1. Since the typedoc output isn't hosted anywhere yet (#113), I repointed it to the canonical Web Client docs page the root README already links to, and relabeled it from "API Reference" to "Web Client docs" so the text matches where it goes.

One line, no content changes. prettier and typos pass (repo has no markdown linter).

On the changelog: doesn't look like it needs a CHANGELOG entry per the CLAUDE.md rules (no API / behavior / breaking change for consumers, plus the "when in doubt, no changelog" note), so I left it out. If a maintainer can add the no changelog label that'll clear the check, or happy to add an entry if you'd prefer.