roles/nginx: Remove 'public' from Cache-Control header

If a max-age is specified the 'public' is implicit.

See: https://developers.google.com/web/fundamentals/performance/optimizing-content-efficiency/http-caching

roles/nginx/templates/wordpress.j2

@@ -7,7 +7,7 @@
     }
 
     location ~* \.(?:ico|css|js|gif|jpe?g|png|svg)$ {
-        add_header Cache-Control "public, max-age=604800";
+        add_header Cache-Control "max-age=604800";
     }   
 
     # Add trailing slash to */wp-admin requests.