[manifest] Update the CSP reference; link directives (#362)

The `manifest-src`directive is defined in [CSP3], not in [CSP2], so I updated the CSP reference. Also linked the related directive definitions, fixed couple of typos.
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/362

-- Commit Summary --

  * Convert the 'Monkey patch' monkey to an HTML entity
  * * Update outdated CSP reference from CSP2 to CSP3
  * Fix typo in img-src directives

-- File Changes --

    M index.html (52)

-- Patch Links --

https://github.com/w3c/manifest/pull/362.patch
https://github.com/w3c/manifest/pull/362.diff

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https://github.com/w3c/manifest/pull/362