fix grammar

Author: KirillKurdyukov

src/Ydb.Sdk/CHANGELOG.md

@@ -1,5 +1,5 @@
 - ADO.NET: Now `YdbConnection.OpenAsync` and `YdbCommand.Execute*` throw `OperationCanceledException`,
-  if the CancellationToken has already been cancelled before the method was called.
+  if the CancellationToken has already been cancelled before the method is called.
 - Feat ADO.NET: decimal type with arbitrary precision/scale ([#498](https://github.com/ydb-platform/ydb-dotnet-sdk/issues/498)).
 - Fixed bug: interval value parsing in microseconds and double instead of ticks ([#497](https://github.com/ydb-platform/ydb-dotnet-sdk/issues/497)).
 - ADO.NET: Changed `IBulkUpsertImporter.AddRowAsync` signature: `object?[] row` → `params object[]`.