Fix rendering of NOTE sections in upgrading.md (#151)

Author: thomasqueirozb

_stable/upgrading.md

@@ -25,6 +25,7 @@ any of hyper's types that have direct backports available.
 
 > **NOTE**: This won't give you warnings about changes where backports were not
 able to be provided.
+
 > **NOTE**: You should upgrade `http-body` to version `0.4.6` or newer to be
 > able to access methods in some suggestions.