default keybinding for bold/italic not invoked

I overrode the default keybinding for bold to use the command key:

```json
    {
        "key": "cmd+b",
        "command": "md-shortcut.toggleBold",
        "when": "editorTextFocus && markdownShortcuts:enabled"
    }
```

If I comment this out and use the default of `ctrl i`, however, I can no longer bold. Seems like the default keybinding does not work.