Fix Arch Linux install command (#11)

pacman -Sy can break the system.

https://wiki.archlinux.org/index.php/System_maintenance#Partial_upgrades_are_unsupported

Author: notfoss

download.html

@@ -48,8 +48,8 @@ <h2 id="linux">Linux</h2>
   </p>
 
   <h3 id="linux-arch">Arch Linux</h3>
-  <p>Arch Linux users can either download Zeal from the community repository:</p>
-  <pre>$ sudo pacman -Sy zeal</pre>
+  <p>Arch Linux users can either install Zeal from the community repository:</p>
+  <pre>$ sudo pacman -Syu zeal</pre>
   <p>Or use the <code>zeal-git</code> AUR package which builds unstable source code from our Git repository.</p>
   <a href="https://aur.archlinux.org/packages/zeal-git/"
      class="btn btn-default" role="button">View zeal-git in AUR</a>