Fix leftover reference to v4.0.0-alpha.6

Running `./build/change-version.js v4.0.0-alpha.6 v4.0.0` fixed this,
so the version change script works fine. I'm presuming instead this
change was just omitted from 35f80bb1, and then wouldn't have
been caught by subsequent runs of `change-version`, since it only
ever replaces the exact old version string specified.

js/src/index.js

@@ -13,7 +13,7 @@ import Util from './util'
 
 /**
  * --------------------------------------------------------------------------
- * Bootstrap (v4.0.0-alpha.6): index.js
+ * Bootstrap (v4.0.0): index.js
  * Licensed under MIT (https://github.com/twbs/bootstrap/blob/master/LICENSE)
  * --------------------------------------------------------------------------
  */