Fix Web support page highlighting in left nav

[wrunk]

The page "Web support for Flutter" wasn't showing an active highlight in the left nav (see screenshots). Looks like there was an extraneous trailing slash.

VERY minor change

Web support for Flutter: No active highlight:

Other pages have:

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[sfshaza2]

/gcbrun

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit f0a4207):

https://flutter-docs-prod--pr11424-wrunk-web-side-nav-fix-4gx5jldw.web.app

[wrunk]

Apologies, my fix was insufficient. For a third level nav item/nav link to get tagged/classed as active, it needs to have 3 path elements (currently only has 2: /platform-integration/web). Options:

• Move the current platform-integration/web/index.md to something else like platform-integration/web/support.md then add in a TOC-like index.md
• Change the permalink for the platform-integration/web drop down from /platform-integration/web -> /platform-integration/web/support but this would likely require a redirect for platform-integration/web
• Something else?

If you let me know your preference I'm happy to update.

[sfshaza2]

@parlough, would you mind owning this one? It looks like some files weren't placed correctly and it's affected the side nav.