fix: add header background if links wrap

Fixes that part of the navigation didn't have a background if the links wrap to the next line.

Author: crisbeto

material.angular.io/src/app/shared/navbar/navbar.scss

@@ -1,9 +1,9 @@
 :host {
   color: var(--mat-sys-primary);
+  background: var(--mat-sys-primary-container);
 }
 
 .docs-navbar-header {
-  background: var(--mat-sys-primary-container);
   display: flex;
   flex-wrap: wrap;
   align-items: center;