Fix typos in sgml docs about RefetchForeignRow().

I screwed this up in ad0bda5d.

Reported-By: Jie Zhang, Michael Paquier, Etsuro Fujita
Discussion: https://postgr.es/m/1396E95157071C4EBBA51892C5368521017F2DA203@G08CNEXMBPEKD02.g08.fujitsu.local

doc/src/sgml/fdwhandler.sgml

@@ -1010,7 +1010,7 @@ RefetchForeignRow(EState *estate,
     </para>
 
     <para>
-     This function should store the tuple into the provided, or clear it if if
+     This function should store the tuple into the provided slot, or clear it if
      the row lock couldn't be obtained.  The row lock type to acquire is
      defined by <literal>erm-&gt;markType</literal>, which is the value
      previously returned by <function>GetForeignRowMarkType</function>.