docs(grid-list): improve basic grid-list example (#3520)

Author: kara

src/examples/grid-list-overview/grid-list-overview-example.css

@@ -1 +1,3 @@
-/** No CSS for this example */
+md-grid-tile {
+  background: lightblue;
+}

src/examples/grid-list-overview/grid-list-overview-example.html

@@ -1,4 +1,4 @@
-<md-grid-list cols="2">
+<md-grid-list cols="2" rowHeight="2:1">
   <md-grid-tile>1</md-grid-tile>
   <md-grid-tile>2</md-grid-tile>
   <md-grid-tile>3</md-grid-tile>

src/examples/grid-list-overview/grid-list-overview-example.ts

@@ -1,8 +1,8 @@
 import {Component} from '@angular/core';
 
-
 @Component({
   selector: 'grid-list-overview-example',
+  styleUrls: ['./grid-list-overview-example.css'],
   templateUrl: './grid-list-overview-example.html',
 })
 export class GridListOverviewExample {}