Update BreakingChanges.md

Author: mckaragoz

BreakingChanges.md

@@ -1,3 +1,10 @@
+### v 6.7.0
+MudStepper's PreventStepChangeAsync func's parameters changed. (A new int target index added)
+Before:
+public Func<StepChangeDirection, Task<bool>> PreventStepChangeAsync
+After:
+public Func<StepChangeDirection, int, Task<bool>> PreventStepChangeAsync
+
 ### v 6.4.0
 Change the css link to:\
 `<link href="_content/CodeBeam.MudBlazor.Extensions/MudBlazor.Extensions.min.css" rel="stylesheet" />`\