fix docs: warning label appearing offscreen

docs/_theme/header.hbs

@@ -35,10 +35,12 @@
   }
 </style>
 <header class='warning'>
+  <p>
     This is the <strong>unpublished</strong> documentation of
     <code>wasm-pack</code>, the published documentation is available
     <a href="https://rustwasm.github.io/docs/wasm-pack/">
       on the main Rust and WebAssembly documentation site
     </a>. Features documented here may not be available in released versions of
     <code>wasm-pack</code>.
+  </p>
 </header>