Fix left sidebar

[srawlins]

Work towards dart-lang/pub-dev#7058

The element ID was wrong. Not sure how long that bug has been around. We don't have web tests. :(

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[srawlins]

CC @isoos

[isoos]

Note: I don't see how this fixes the issue on pub.dev, as we did created the dartdoc-sidebar-left-content.

[srawlins]

Thanks for noting that; changed from "fixes" to "work towards".