Fix links in the header.

These link destinations are on the main Swift.org website, not on
docs.swift.org, so they can't use a relative URL.

Author: amartini51

TSPL.docc/header-publish.html

@@ -21,11 +21,11 @@
     <button id="small-menu-toggle"></button>
     <nav id="menu">
       <ul class="links">
-        <li><a href="/blog/">Blog</a></li>
-        <li><a href="/download/">Download</a></li>
-        <li><a href="/getting-started/">Getting Started</a></li>
-        <li><a href="/documentation/">Documentation</a></li>
+        <li><a href="https://www.swift.org/blog/">Blog</a></li>
+        <li><a href="https://www.swift.org/download/">Download</a></li>
+        <li><a href="https://www.swift.org/getting-started/">Getting Started</a></li>
+        <li><a href="https://www.swift.org/documentation/">Documentation</a></li>
       </ul>
     </nav>
   </div>
-</header>
+</header>