Fix typos and Markdown formatting in the explainer docs

[lazizbekravshanov]

Small editorial fixes across the README, the declarative API explainer, and the service worker explainer. No content changes.

• README: an inline code span opens before registerTool() and never closes, so the rest of the heading renders as code.
• Declarative explainer: a bold marker on the toolactivated and toolcanceled heading is never closed; two i.e. that should be e.g., matching the imperative example directly above them; subject and verb agreement on the TBD sentence and on submit event handlers; a stray article in "no such a tag".
• Service worker explainer: a capital after a semicolon, a missing possessive in "the LLMs limited context window", and subject and verb agreement in "it reads/write to".

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domfarolino marked as non substantive for IPR from ash-nazg.