[css-grid] Fix typo: gridDefinitionColumns vs gridTemplateColumns

Author: mrego

css-grid/Overview.bs

@@ -1248,7 +1248,7 @@ Resolved Values</h4>
 			&lt;/div>
 			&lt;script>
 			  var gridElement = document.getElementById("grid");
-			  getComputedStyle(gridElement).gridDefinitionColumns;
+			  getComputedStyle(gridElement).gridTemplateColumns;
 			  // (a) 50px (b) 320px (b c d) repeat(2, (e) 40px) repeat(4, 0px) 50px
 			&lt;/script>
 		</pre>