Update starlette subdomain in docs

[alDuncanson]

Fix broken link to Starlette site with www subdomain.

[hyperlint-ai]

PR Change Summary

Updated the link to the Starlette site to use the correct www subdomain.

• Fixed broken link to Starlette site by updating to www subdomain.

Modified Files

• docs/a2a.md

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[DouweM]

@alDuncanson Thanks Al!