Additional info added to fifth step (SharePoint#2084)

* Additional info added to fifth step

In the 5th step, its better we explicitly call out to do not select 'y' for the "Do you want to allow tenant admin..." question while scaffolding the project. In the 3rd article (how to deploy extension to sharepoint), it uses the elements.xml file which will be missing if user selects 'y' for this question.

* Update build-a-hello-world-extension.md

Author: svarukala

docs/spfx/extensions/get-started/build-a-hello-world-extension.md

@@ -51,6 +51,7 @@ You can also follow the steps in this article by watching the video on the Share
 
     * Accept the default **HelloWorld** as your extension name, and select Enter.
     * Accept the default **HelloWorld description** as your extension description, and select Enter.
+    * For the next question **Do you want to allow the tenant admin the choice of being able to deploy the solution to all sites....**, ensure you select No (N) , and select Enter. If you select Yes (y), the scaffolding will not generate the Elements.xml feature deployment file.
 
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