pgsql: Minor improvement in documentation of temp_tablespaces: avoid

Log Message:
-----------
Minor improvement in documentation of temp_tablespaces: avoid grammatical
ambiguity and say what the default value is.

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.155 -> r1.156)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.155&r2=1.156)