BUGFIX: Keybinding was not restricted to the yUML context.

My apologies, I did not realize that keybindings defined within a plugin are not restricted to the scope of the plugin and therefore the keybinding that I added was interfering with `super+b` outside of the context of the yUML plugin.

To address this, I added a context restriction so that the keybinding is only active when editing yUML files (selector: source.yuml)