Fixed formatting toolbar source link (#620)

Author: matthewlipski

docs/pages/docs/ui-components/formatting-toolbar.mdx

@@ -34,7 +34,7 @@ Setting `formattingToolbar={false}` on `BlockNoteView` tells BlockNote not to sh
         <strong>Tip:</strong> The children you pass to the `FormattingToolbar` component
         should be default dropdowns/buttons (e.g. `BlockTypeDropdown` & `BasicTextStyleButton`) or custom dropdowns/buttons
         (`ToolbarDropdown` & `ToolbarButton`). To see all the components you can use, head to the
-        [Formatting Toolbar's source code](link).
+        [Formatting Toolbar's source code](https://github.com/TypeCellOS/BlockNote/blob/main/packages/react/src/components/FormattingToolbar/mantine/FormattingToolbar.tsx).
     </small>
 </div>