Changelog entry for not-quite-so-important behavior change

Task-number: QTBUG-30837
Change-Id: Ia39c0d85d29cf87861083e4a5170aab701fb2199
Reviewed-by: Andrew den Exter <andrew.den.exter@qinetic.com.au>

dist/changes-5.1.0

@@ -46,6 +46,8 @@ Third party components
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 *                          Library                                         *
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+ - QTBUG-30837: The Flickable type no longer fixes up the content area on
+   startup to move it inside the viewport.
 
 
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