[mustache_template] Fix broken README link

[stuartmorgan-g]

pub.dev flagged this link since it used http rather than https, but it turns out it's also no longer a valid link; that domain doesn't resolve. This updates to the correct domain, and to a secure link.

Part of flutter/flutter#183844

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request updates a broken link to the Mustache manual in the README.md file. The package version has been incremented to 2.0.4 in pubspec.yaml, and a corresponding entry has been added to CHANGELOG.md to reflect this change.