Fixed a wrong command (SharePoint#1187)

Fixed a wrong command **Get-PnPAvailableApp** with the right one **Get-PnPApp**

Author: giuleon

docs/apis/alm-api-for-spfx-add-ins.md

@@ -182,5 +182,5 @@ Uninstall-PnPApp -Identity <app id>
 Getting a list of apps that can be added to the site is possible using:
 
 ```PowerShell
-Get-PnPAvailableApp
+Get-PnPApp
 ```