BUG #11973: duplicate supported platforms

The following bug has been logged on the website:

Bug reference:      11973
Logged by:          KOIZUMI Satoru
Email address:      koizumistr@minos.ocn.ne.jp
PostgreSQL version: 9.4beta2
Operating system:   KOIZUMI Satoru
Description:

FreeBSD, OpenBSD, NetBSD, OS X, AIX, HP/UX and Solaris appear twice in the
following sentence in "15.6. Supported Platforms" of PostgreSQL 9.4beta3
Documentation.

PostgreSQL can be expected to work on these operating systems: Linux (all
recent distributions), Windows (Win2000 SP4 and later), FreeBSD, OpenBSD,
NetBSD, OS X, AIX, HP/UX, Solaris, Tru64 Unix, FreeBSD, OpenBSD, NetBSD, OS
X, AIX, HP/UX, Solaris, and UnixWare.