Doc: fix ancient mistake in search_path documentation.

"$user" in a search_path string is replaced by CURRENT_USER not
SESSION_USER.  (It actually was SESSION_USER in the initial implementation,
but we changed it shortly later, and evidently forgot to fix the docs to
match.)

Noted by antonov@stdpr.ru

Discussion: https://postgr.es/m/159151fb45d490c8d31ea9707e9ba99d@stdpr.ru

Author: tglsfdc

doc/src/sgml/config.sgml

@@ -5946,7 +5946,7 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
        <para>
         If one of the list items is the special name
         <literal>$user</literal>, then the schema having the name returned by
-        <function>SESSION_USER</> is substituted, if there is such a schema
+        <function>CURRENT_USER</> is substituted, if there is such a schema
         and the user has <literal>USAGE</> permission for it.
         (If not, <literal>$user</literal> is ignored.)
        </para>