Update changelog for preview release

Author: markcowl

src/Batch/Batch/ChangeLog.md

@@ -17,8 +17,9 @@
     * Overview of change #1
         - Additional information about change #1
 -->
-
 ## Upcoming Release
+
+## 2.0.0-preview
 * Renamed `CoreQuota` on `BatchAccountContext` to `DedicatedCoreQuota`. There is also a new `LowPriorityCoreQuota`.
   - This impacts **Get-AzBatchAccount**.
 * **New-AzBatchTask** `-ResourceFile` parameter now takes a collection of `PSResourceFile` objects, which can be constructed using the new **New-AzBatchResourceFile** cmdlet.