Themes: Replace `—` with `–` as a separator between Monday and...

Themes: Replace `—` with `–` as a separator between Monday and Friday in theme starter content.

The `ndash` standardly is the "through" indicator for showing inclusive dates and numbers, while the `mdash` is an interruptor and used as an aside.

Props mikejdent, garrett-eclipse.
Fixes #48286.
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git-svn-id: https://core.svn.wordpress.org/trunk@46934 1a063a9b-81f0-0310-95a4-ce76da25c4cd