default branch changed to preview

CONTRIBUTING.md

@@ -74,7 +74,7 @@ You can find all of the pull requests that have been opened in the [Pull Request
 To open your own pull request, click [here](https://github.com/Azure/azure-powershell/compare). When creating a pull request, keep the following in mind:
 - Make sure you are pointing to the fork and branch that your changes were made in
 - Choose the correct branch you want your pull request to be merged into
-    - The **dev** branch is for active development; changes in this branch will be in the next Azure PowerShell release
+    - The **preview** branch is for active development; changes in this branch will be in the next Azure PowerShell release
     - The **master** branch contains a snapshot of the source code at the time of the last release
     - The **release-X.X.X** branch is for active development during a release
 - The pull request template that is provided **should be filled out**; this is not something that should just be deleted or ignored when the pull request is created