FIX: update external link in top navigation target (#1145)

### Description of Changes
this updates the `nav-menu-item` with external links to have
`target="_bank"` instead of `target="blank"`. There's [a slight
difference in the behavior](https://css-tricks.com/targetblank/).
and other external links have used the underscore.

app/scripts/components/common/page-header/nav-menu-item.tsx

@@ -100,7 +100,7 @@ export default function NavMenuItem({ item, alignment, onClick }: {item: NavItem
       <li key={`${title}-nav-item`}>
       <GlobalMenuLink 
         as='a'
-        target='blank'
+        target='_blank'
         rel='noopener'
         onClick={onClick}
         href={(rest as ExternalNavLink).href}