Fix changelog (#2339)

# Pull Request Checklist

Please leverage this checklist as a reminder to address commonly occurring feedback when submitting a pull request to make sure your PR can be reviewed quickly:

See the detailed list in the [contributing guide](https://github.com/Azure/azure-sdk-for-cpp/blob/master/CONTRIBUTING.md#pull-requests).

- [x] [C++ Guidelines](https://azure.github.io/azure-sdk/cpp_introduction.html)
- [x] Doxygen docs
- [x] Unit tests
- [x] No unwanted commits/changes
- [x] Descriptive title/description
  - [x] PR is single purpose
  - [x] Related issue listed
- [x] Comments in source
- [x] No typos
- [x] Update changelog
- [x] Not work-in-progress
- [x] External references or docs updated
- [x] Self review of PR done
- [x] Any breaking changes?

sdk/storage/azure-storage-files-datalake/CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## 12.0.0-beta.12 (Unreleased)
 
+### Breaking Changes
+
+- Renamed `ContentLength` in `FlushFileResult` to `FileSize`.
 
 ## 12.0.0-beta.11 (2021-05-19)
 
@@ -15,7 +18,6 @@
 - Renamed `HasMorePages()` in paged response to `HasPage()`.
 - Default chunk size for concurrent upload was changed to nullable.
 - `DataLakeLeaseClient::Change()` updates internal lease id.
-- Renamed `ContentLength` in `FlushFileResult` to `FileSize`.
 
 ## 12.0.0-beta.10 (2021-04-16)
 

sdk/storage/azure-storage-files-shares/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## 12.0.0-beta.12 (Unreleased)
 
+### Breaking Changes
+
+- Renamed `ContentLength` in `FileItemDetails` to `FileSize`.
+
 
 ## 12.0.0-beta.11 (2021-05-19)
 
@@ -15,7 +19,6 @@
 - Renamed `HasMorePages()` in paged response to `HasPage()`.
 - `ShareLeaseClient::Change()` updates internal lease id.
 - `ShareItem::ShareMetadata` was renamed to `ShareItem::Metadata`.
-- Renamed `ContentLength` in `FileItemDetails` to `FileSize`.
 
 ## 12.0.0-beta.10 (2021-04-16)