fix markdown cell focus indicator bottom (microsoft#184544)

Author: rebornix

src/vs/workbench/contrib/notebook/browser/media/notebook.css

@@ -419,6 +419,7 @@
 
 /** Selected cell background */
 .notebookOverlay .monaco-list.selection-multiple .markdown-cell-row.selected,
+.notebookOverlay .monaco-list.selection-multiple .markdown-cell-row.selected .cell-focus-indicator-bottom,
 .notebookOverlay .monaco-list.selection-multiple .code-cell-row.selected .cell-focus-indicator-top,
 .notebookOverlay .monaco-list.selection-multiple .code-cell-row.selected .cell-focus-indicator-left,
 .notebookOverlay .monaco-list.selection-multiple .code-cell-row.selected .cell-focus-indicator-right,