Bug 574684 - Add help stubs for 4.21

Point to 4.21 content in welcome page.

Change-Id: I7cd6948f9e07b6918a605f7d40e69cc454dee927
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform/+/182809
Tested-by: Alexander Kurtakov <akurtako@redhat.com>
Reviewed-by: Alexander Kurtakov <akurtako@redhat.com>
diff --git a/platform/org.eclipse.platform/intro/migrateExtensionContent.xml b/platform/org.eclipse.platform/intro/migrateExtensionContent.xml
index 2df559e..a90faca 100644
--- a/platform/org.eclipse.platform/intro/migrateExtensionContent.xml
+++ b/platform/org.eclipse.platform/intro/migrateExtensionContent.xml
@@ -3,8 +3,8 @@
   <!-- Extension to the SDK Migration Page. -->
   <extensionContent id="org.eclipse.platform" name="Eclipse Platform" alt-style="css/migrate.properties" style="css/migrate.css" path="migrate/@">
      <group id="content-group" style-id="content-group">
-    	<link style-id="content-link" label="Eclipse Platform" url="http://org.eclipse.ui.intro/showHelpTopic?id=/org.eclipse.platform.doc.isv/porting/eclipse_4_20_porting_guide.html" id="platform">
-      		<text>Eclipse Platform 4.20 Plug-in Migration Guide</text>
+    	<link style-id="content-link" label="Eclipse Platform" url="http://org.eclipse.ui.intro/showHelpTopic?id=/org.eclipse.platform.doc.isv/porting/eclipse_4_21_porting_guide.html" id="platform">
+      		<text>Eclipse Platform 4.21 Plug-in Migration Guide</text>
     	</link>
      </group>
   </extensionContent>