Fix a typo

PR:		252800

en_US.ISO8859-1/books/handbook/disks/chapter.xml

@@ -2901,7 +2901,7 @@ Filesystem     Size   Used  Avail Capacity  Mounted on
       <screen>&prompt.root; <userinput>umount /private</userinput>
 &prompt.root; <userinput>geli detach da2.eli</userinput></screen>
 
-      <para>A <filename>rc.d</filename> script is provided to
+      <para>An <filename>rc.d</filename> script is provided to
 	simplify the mounting of <command>geli</command>-encrypted
 	devices at boot time.  For this example, add these lines to
 	<filename>/etc/rc.conf</filename>:</para>