fixed changelog as per feedback and updated migration guide

Signed-off-by: Dharini Dutia <[email protected]>

Changelog.md

@@ -3,16 +3,16 @@
 ### Gazebo Sim 6.12.0 (2022-08-30)
 
 1. Add topic parameter to thrust plugin
- * [Pull request #1681](https:/gazebosim/gz-gazebo/pull/1681)
+ * [Pull request #1681](https:/gazebosim/gz-sim/pull/1681)
 
-1. Add information about `<topic>` system paramter
- * [Pull request #1671](https:/gazebosim/gz-gazebo/pull/1671)
+1. Add information about `<topic>` system parameter
+ * [Pull request #1671](https:/gazebosim/gz-sim/pull/1671)
 
 1. Adding tests for hydrodynamics
- * [Pull request #1617](https:/gazebosim/gz-gazebo/pull/1617)
+ * [Pull request #1617](https:/gazebosim/gz-sim/pull/1617)
 
 1. Fix Windows and Doxygen
- * [Pull request #1643](https:/gazebosim/gz-gazebo/pull/1643)
+ * [Pull request #1643](https:/gazebosim/gz-sim/pull/1643)
 
 ### Gazebo Sim 6.11.0 (2022-08-17)
 

Migration.md

@@ -5,6 +5,12 @@ Deprecated code produces compile-time warnings. These warning serve as
 notification to users that their code should be upgraded. The next major
 release will remove the deprecated code.
 
+## Ignition Gazebo 6.11.X to 6.12.X
+
+ * **Modified**:
+ + In the Hydrodynamics plugin, inverted the added mass contribution to make it
+ act in the correct direction.
+
 ## Ignition Gazebo 6.1 to 6.2
 
 * If no `<namespace>` is given to the `Thruster` plugin, the namespace now