pgsql: In docs, rename "backwards compatibility" to "backward compatibi

In docs, rename "backwards compatibility" to "backward compatibility"
for consistency.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/7a8f43968add3c69b79c49ef236d945e643dcb1e

Modified Files
--------------
doc/src/sgml/array.sgml      |    2 +-
doc/src/sgml/config.sgml     |    2 +-
doc/src/sgml/func.sgml       |    2 +-
doc/src/sgml/libpq.sgml      |    4 ++--
doc/src/sgml/lobj.sgml       |    2 +-
doc/src/sgml/plperl.sgml     |    2 +-
doc/src/sgml/user-manag.sgml |    2 +-
doc/src/sgml/xfunc.sgml      |    4 ++--
8 files changed, 10 insertions(+), 10 deletions(-)