Fix typo in doc.

unix_socket_directories was introduced in 9.3, but the document
in older versions wrongly have mentioned it. This commit replaces
it with the correct older name unix_socket_directory.

This is applied to only 9.2 and older supported versions.

Guillaume Lelarge

Author: MasaoFujii

doc/src/sgml/config.sgml

@@ -509,7 +509,7 @@ SET ENABLE_SEQSCAN TO OFF;
        <para>
         This parameter is irrelevant on systems, notably Solaris as of Solaris
         10, that ignore socket permissions entirely.  There, one can achieve a
-        similar effect by pointing <varname>unix_socket_directories</> to a
+        similar effect by pointing <varname>unix_socket_directory</> to a
         directory having search permission limited to the desired audience.
         This parameter is also irrelevant on Windows, which does not have
         Unix-domain sockets.