docs: use stable Amplitude logo URL

[aliaksandr-kazarez]

Summary

Replace the README logo URL tied to an old frontend build with the stable Amplitude brand URL. This prevents the logo from depending on retention of that build asset.

Related: SDK-353.

Validation

• Verified that the diff contains only exact logo URL substitutions and no old URL remains in tracked files.
• git diff --check passed.
• Runtime tests not run: documentation-only URL change.

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?: No; no runtime or public API changes.

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Note

Low Risk
Documentation-only README image URL swap with no runtime or API impact.

Overview
Updates the README header image to use the stable Amplitude brand asset at https://static.amplitude.com/brand/amplitude-logo-with-text.svg instead of a hashed path under an old frontend build (lightning/.../amplitude-logo-with-text.4fb9e463.svg).

No SDK code, APIs, or behavior change—only the logo src URL so the README does not break if that legacy static build is removed.

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[linear-code]

SDK-353