filter/constants: fix FILTER_FLAG_STRIP_BACKTICK description

Author: Ayesh

reference/filter/constants.xml

@@ -189,7 +189,7 @@
    </term>
    <listitem>
     <simpara>
-     Strips backtick (<literal>`</literal> characters.
+     Strips backtick (<literal>`</literal>) characters.
     </simpara>
    </listitem>
   </varlistentry>