Create using-page-placeholder-with-extensions.md (SharePoint#620)

Author: mjimison

docs/spfx/extensions/get-started/using-page-placeholder-with-extensions.md

@@ -218,9 +218,9 @@ contoso.sharepoint.com/Lists/Contoso/AllItems.aspx?loadSPFX=true&debugManifestsF
 
 Click "**Load debug scripts**" button to continue loading scripts from your local host.
 
-You should now see the alert message in your page. 
+You should now see the custom header and footer content in your page. 
 
 ![Custom header and footer elements rendered in the page](../../../../images/ext-app-header-footer-visible.png)
 
 ## Next steps
-Congratulations on building your own custom header and footer from Application Customizer. You can continue building out your Hello World Extension in the next topic [Deploy your extension to site collection (Hello world part 3)](./serving-your-extension-from-sharepoint.md). You will learn how to deploy and preview the Hello World extension in SharePoint site collection without using **Debug** query parameters. 
+Congratulations on building your own custom header and footer from Application Customizer. You can continue building out your Hello World Extension in the next topic [Deploy your extension to site collection (Hello world part 3)](./serving-your-extension-from-sharepoint.md). You will learn how to deploy and preview the Hello World extension in SharePoint site collection without using **Debug** query parameters.