Add serial comma for clarity

Signed-off-by: Roger Sheen <[email protected]>

index.dita

@@ -54,7 +54,7 @@
             conkeyref="pdf-customization/shortdesc-ph"
           />.</li>
         <li outputclass="list-group-item list-group-item-action">
-          <xref keyref="adding-plugins" outputclass="stretched-link"/> explains how to install, remove and discover new
+          <xref keyref="adding-plugins" outputclass="stretched-link"/> explains how to install, remove, and discover new
           plug-ins.</li>
         <li outputclass="list-group-item list-group-item-action">
           <xref keyref="custom-plugins" outputclass="stretched-link"/> provides information on extending DITA-OT with