doc: FreeBSD uses camcontrol identify, not atacontrol, for cache

This is for IDE drive cache control, same as SCSI (already documented
properly).

Reported-by: John Ekins

Discussion: https://postgr.es/m/20170808224017.8424.69170@wrigleys.postgresql.org

Author: John Ekins

Backpatch-through: 12

doc/src/sgml/wal.sgml

@@ -86,7 +86,7 @@
     <listitem>
       <para>
         On <productname>FreeBSD</productname>, IDE drives can be queried using
-        <command>atacontrol</command> and write caching turned off using
+        <command>camcontrol identify</command> and write caching turned off using
         <literal>hw.ata.wc=0</literal> in <filename>/boot/loader.conf</filename>;
         SCSI drives can be queried using <command>camcontrol identify</command>,
         and the write cache both queried and changed using