Small Doc improvements: default value for enableInteractiveSelection.

[greymag]

Small Doc improvement: default value for EditableText.enableInteractiveSelection.

I didn't create an issue because it's just one line docs improvement. I can create one if you say so.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
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• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
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[flutter-dashboard]

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[goderbauer]

LGTM