Minor docs tweaks

docs/topics/3.0-announcement.md

@@ -34,7 +34,7 @@ Significant new functionality continues to be planned for the 3.1 and 3.2 releas
 
 This talk from the [Django: Under the Hood](http://www.djangounderthehood.com/) event in Amsterdam, Nov 2014, gives some good background context on the design decisions behind 3.0.
 
-<iframe width="560" height="315" src="//www.youtube.com/embed/3cSsbe-tA0E" frameborder="0" allowfullscreen></iframe>
+<iframe style="display: block; margin: 0 auto 0 auto" width="560" height="315" src="//www.youtube.com/embed/3cSsbe-tA0E" frameborder="0" allowfullscreen></iframe>
 
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