Fix NotebookDocumentBackup comment

src/vs/vscode.proposed.d.ts

@@ -1673,7 +1673,7 @@ declare module 'vscode' {
 		/**
 		 * Unique identifier for the backup.
 		 *
-		 * This id is passed back to your extension in `openCustomDocument` when opening a notebook editor from a backup.
+		 * This id is passed back to your extension in `openNotebook` when opening a notebook editor from a backup.
 		 */
 		readonly id: string;