test(guide/expression): we must tell protractor to wait for the alert…

… appear

This is a better solution than f91eb0e

docs/content/guide/expression.ngdoc

@@ -142,7 +142,7 @@ provide mockable access to globals.
       element(by.css('[ng-click="greet()"]')).click();
 
       // We need to give the browser time to display the alert
-      browser.sleep(100);
+      browser.wait(protractor.ExpectedConditions.alertIsPresent(), 1000);
 
       var alertDialog = browser.switchTo().alert();