Fixes a bug that was preventing users from setting their own taglines.

In the theme files, the header was set to:
    <h1>{{ page.title }} <small>Supporting Tagline</small></h1>
Becasue the tagline was set to "Supporting Tagline", users could not remove or set a custom tagline.
Changing `Supporting Tagline` to `{{ page.tagline }}` is sufficient to fix this.