docs: update gantt zoom demo cover

Author: fangsmile

docs/assets/demo/en/gantt/gantt-datazoom.md

@@ -2,7 +2,7 @@
 category: examples
 group: gantt
 title: Gantt DataZoomAxis Scrollbar
-cover:
+cover: https://lf9-dp-fe-cms-tos.byteorg.com/obj/bit-cloud/VTable/preview/gantt-zoom-axis.gif
 link: gantt/datazoom
 option: Gantt#zoomScale.dataZoomAxis
 ---

docs/assets/demo/en/gantt/gantt-zoom.md

@@ -2,7 +2,7 @@
 category: examples
 group: gantt
 title: Gantt Chart Smart Zoom Feature
-cover:
+cover: https://lf9-dp-fe-cms-tos.byteorg.com/obj/bit-cloud/VTable/preview/gantt-zoom.gif
 link: gantt/zoom-scale
 option: Gantt#zoomScale
 ---

docs/assets/demo/zh/gantt/gantt-datazoom.md

@@ -2,7 +2,7 @@
 category: examples
 group: gantt
 title: 甘特图 DataZoomAxis 滚动条
-cover:
+cover: https://lf9-dp-fe-cms-tos.byteorg.com/obj/bit-cloud/VTable/preview/gantt-zoom-axis.gif
 link: gantt/datazoom
 option: Gantt#zoomScale.dataZoomAxis
 ---

docs/assets/demo/zh/gantt/gantt-zoom.md

@@ -2,7 +2,7 @@
 category: examples
 group: gantt
 title: 甘特图智能缩放功能
-cover:
+cover: https://lf9-dp-fe-cms-tos.byteorg.com/obj/bit-cloud/VTable/preview/gantt-zoom.gif
 link: gantt/zoom-scale
 option: Gantt#zoomScale
 ---