Update HTOS wording as the book is no longer "upcoming"

Author: schneems

app/views/layouts/mail_layout.html.erb

@@ -13,7 +13,7 @@
     <div class="footer-links" style="padding: 0">
 
       <p>
-        <strong>Don't want to receive free content from my upcoming book <dfn>"How to Open Source"</dfn>?</strong>
+        <strong>Don't want to receive content or announcements from my book <dfn>"How to Open Source"</dfn>?</strong>
         <%= link_to "Click here to 'mute' these emails", edit_user_url(@user) %>.
         You'll still receive CodeTriage emails.
       </p>