Cookbook: Change "equal" to "assert.equal" in example

In cookbook example, global "equal" method appears to be typo in
relation to more prevalent "assert.equal" usage in cookbook. Comments
in qunit-1.15.0.js source also indicate global "equal" is "deprecated"
and remains only for "backwards compatibility."

Ref qunitjs/qunit#228

pages/cookbook.html

@@ -341,7 +341,7 @@ <h3 class="title" id="solution-165">Solution</h3>
 	var $fixture = $( "#qunit-fixture" );
 
 	$fixture.append( "&lt;div>hello!&lt;/div>" );
-	equal( $( "div", $fixture ).length, 1, "div added successfully!" );
+	assert.equal( $( "div", $fixture ).length, 1, "div added successfully!" );
 });
 
 QUnit.test( "Appends a span", function( assert ) {