prevent version select being autocompleted by the browser (#1225)

This fixes an issue where when you navigate back in history, the
browser pre-fills the select with the value from just after you
navigated.

Repro:

1. Navigate to lit.dev.
2. Select `v1`, and be navigated to the v1 docs.
3. Press back in history, and note the select is set to v1.

packages/lit-dev-content/site/_includes/site-nav.html

@@ -2,7 +2,7 @@
    <a href="{{ site.baseurl }}/docs/">Documentation</a>
 
    <litdev-version-selector>
-      <select name="version">
+      <select name="version" autocomplete="off">
         {% for version, versionData in versions %}
           <option value="{{ version }}"
             {% if version == selectedVersion %}