docs: fix formatting of open option - closes BrowserSync/browsersync.github.io#50

Author: shakyShane

lib/default-config.js

@@ -255,7 +255,7 @@ module.exports = {
 
     /**
      * Decide which URL to open automatically when Browsersync starts. Defaults to "local" if none set.
-     * Can be true, `local`, `external`, `ui`, `ui-external`, `tunnel` or `false`
+     * Can be `true`, `local`, `external`, `ui`, `ui-external`, `tunnel` or `false`
      * @property open
      * @type Boolean|String
      * @default true