Widgets: Adjust formatting for displaying the closing `</nav>` tag in widgets...

Widgets: Adjust formatting for displaying the closing `</nav>` tag in widgets for consistency with the opening tag.

Follow-up to [48349].

See #48170.
Built from https://develop.svn.wordpress.org/trunk@48410


git-svn-id: https://core.svn.wordpress.org/trunk@48179 1a063a9b-81f0-0310-95a4-ce76da25c4cd