Rename link to book "Learn" since that is the action to take

And it isn't otherwise clear what the book might be from context

Author: fitzgen

Diff for: _includes/header.html

@@ -22,7 +22,7 @@
               </label>
 
               <div class="trigger">
-                  <a class="page-link" href="https://rustwasm.github.io/book/">📚 Book</a>
+                  <a class="page-link" href="https://rustwasm.github.io/book/">📚 Learn</a>
                   <a class="page-link" href="https://github.com/rustwasm/team/blob/master/README.md#get-involved">👯 Get Involved</a>
                 {%- for path in page_paths -%}
                   {%- assign my_page = site.pages | where: "path", path | first -%}