doc: Fix incorrect wording for --file in pg_dump

The documentation stated that the directory specified by --file
must not exist, but pg_dump does allow for empty directories to
be specified and used.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Bruce Momjian <bruce@momjian.us>
Discussion: https://postgr.es/m/534AA60D-CF6B-432F-9882-E9737B33D1B7@gmail.com

doc/src/sgml/ref/pg_dump.sgml

@@ -297,8 +297,8 @@ PostgreSQL documentation
         file based output formats, in which case the standard output is used.
         It must be given for the directory output format however, where it
         specifies the target directory instead of a file. In this case the
-        directory is created by <command>pg_dump</command> and must not exist
-        before.
+        directory is created by <command>pg_dump</command> unless the directory
+        exist and is empty.
        </para>
       </listitem>
      </varlistentry>