docs: update Fastly package.json description

[mmrj]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

https://launchdarkly.atlassian.net/browse/DOCS-973

Describe the solution you've provided

package.json description field for Fastly is incorrect, this PR updates it

Describe alternatives you've considered

n/a

Additional context

I'm not sure what, if anything, needs to happen after this update (is a patch release required for this to get picked up anywhere?)

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[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
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Size limit: 21000