Fix Memcached::RES_UNKNOWN_STAT_KEY constant label

Follow-up to #3873.

Author: Ayesh

reference/memcached/constants.xml

@@ -842,7 +842,7 @@
   </varlistentry>
 
   <varlistentry xml:id="memcached.constants.res-unknown-stat-key">
-   <term><constant>Memcached::RES_BAD_KEY_PROVIDED</constant></term>
+   <term><constant>Memcached::RES_UNKNOWN_STAT_KEY</constant></term>
    <listitem>
     <simpara>Unknown stat key.</simpara>
    </listitem>