Update migrate-application-authentication-to-azure-active-directory.md

Add heading markup for "Legacy apps that you choose to modernize" on line 268

Author: miloshb

articles/active-directory/manage-apps/migrate-application-authentication-to-azure-active-directory.md

@@ -265,7 +265,7 @@ The already modernized apps are the most likely to be moved to Azure AD. These a
 
 In addition to the choices in the [Azure AD app gallery,](https://azuremarketplace.microsoft.com/marketplace/apps/category/azure-active-directory-apps) these could be apps that already exist in your organization or any third-party apps from a vendor who is not a part of the Azure AD gallery ([non-gallery applications)](./add-application-portal.md).
 
-Legacy apps that you choose to modernize
+### Legacy apps that you choose to modernize
 
 For legacy apps that you want to modernize, moving to Azure AD for core authentication and authorization unlocks all the power and data-richness that the [Microsoft Graph](https://developer.microsoft.com/graph/gallery/?filterBy=Samples,SDKs) and [Intelligent Security Graph](https://www.microsoft.com/security/operations/intelligence?rtc=1) have to offer.