Adapt ext point description for onboarding image (eclipse-platform#721)

* Adapt ext point description for onboarding image

Size shall be 250x250.

Issue: eclipse-platform#702

* Update bundles/org.eclipse.ui/schema/perspectiveExtensions.exsd

of course...

Co-authored-by: BeckerWdf <ma.becker@sap.com>

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Co-authored-by: BeckerWdf <ma.becker@sap.com>

bundles/org.eclipse.ui/schema/perspectiveExtensions.exsd

@@ -103,8 +103,8 @@ are appended to the initial definition.
             <annotation>
                <documentation>
                   The image is shown in the editor area in case no editor is open.
-The image shall be grey and not colored and shall have a size of 350 x 350 px.
-Plus a second image for high resolution with a size of 700 x 700 px and a name like [image_name]@2x.[image_type].
+The image shall be grey and not colored and shall have a size of 250 x 250 px.
+Plus a second image for high resolution with a size of 500 x 500 px and a name like [image_name]@2x.[image_type].
                </documentation>
                <appInfo>
                   <meta.attribute kind="resource"/>