Reduce typename width to 6.25rem

This makes "existential type" look slightly cramped (though still
readable), but it makes all other typenames look better. Existential
types are currently very rare, and we can always tweak this later if
necessary.

src/librustdoc/html/static/css/rustdoc.css

@@ -894,7 +894,7 @@ so that we can apply CSS-filters to change the arrow color in themes */
 	display: inline-block;
 	color: var(--search-results-grey-color);
 	font-size: 0.875rem;
-	width: 7rem;
+	width: 6.25rem;
 }
 
 .popover {