move items to improvements in 19.3 release notes

Author: tony-xlh

_articles/info/schedule/Stable.md

@@ -24,6 +24,9 @@ description: Dynamic Web TWAIN SDK Documentation Schedule Stable Release Page
 * Added prevention of service installation on macOS versions lower than 10.15.
 * Added support for service installation for multiple users (Windows only).
 * Added an [`EnableLocalNetworkMixedContent`](/_articles/info/api/Dynamsoft_WebTwainEnv.md#enablelocalnetworkmixedcontent) property.
+
+### Improvements
+
 * Improved the performance of [`RemoveAllSelectedImages()`](/_articles/info/api/WebTwain_Buffer.md#removeallselectedimages) by triggering [`OnBufferChanged`](/_articles/info/api/WebTwain_Buffer.md#onbufferchanged) and [`OnBitmapChanged`](/_articles/info/api/WebTwain_Buffer.md#onbitmapchanged) events only once.
 * Improved performance for the viewer loading documents with thousands of pages.
 * Updated third-party libraries to enhance security.