migration.md: fix Jekyll variable. (twbs#24449)

Rationale: the page is not available locally, only on gh-pages, thus leading to a 404 locally.

Author: XhmikosR

docs/4.0/migration.md

@@ -130,7 +130,7 @@ New to Bootstrap 4 is the [Reboot]({{ site.baseurl }}/docs/{{ site.docs_version
 
 - Renamed `.btn-default` to `.btn-secondary`.
 - Dropped the `.btn-xs` class entirely as `.btn-sm` is proportionally much smaller than v3's.
-- The [stateful button]({{ site.baseurl }}/docs/3.3/javascript/#buttons-stateful) feature of the `button.js` jQuery plugin has been dropped. This includes the `$().button(string)` and `$().button('reset')` methods. We advise using a tiny bit of custom JavaScript instead, which will have the benefit of behaving exactly the way you want it to.
+- The [stateful button]({{ site.url }}/docs/3.3/javascript/#buttons-stateful) feature of the `button.js` jQuery plugin has been dropped. This includes the `$().button(string)` and `$().button('reset')` methods. We advise using a tiny bit of custom JavaScript instead, which will have the benefit of behaving exactly the way you want it to.
   - Note that the other features of the plugin (button checkboxes, button radios, single-toggle buttons) have been retained in v4.
 - Change buttons' `[disabled]` to `:disabled` as IE9+ supports `:disabled`. However `fieldset[disabled]` is still necessary because [native disabled fieldsets are still buggy in IE11](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/fieldset#Browser_compatibility).