Added missing keybind in linux.cson.

`'ctrl-,': 'application:show-settings'` exists in `win32.cson` and `darwin32.cson`

keymaps/linux.cson

@@ -14,6 +14,7 @@
   'F11': 'window:toggle-full-screen'
 
   # Sublime Parity
+  'ctrl-,': 'application:show-settings'
   'ctrl-N': 'application:new-window'
   'ctrl-W': 'window:close'
   'ctrl-o': 'application:open-file'