Fix duplicated platforms due to copy/paste error

Patch from Michael Paquier, mistake spotted by KOIZUMI Satoru

Author: mhagander

doc/src/sgml/installation.sgml

@@ -2002,7 +2002,6 @@ kill `cat /usr/local/pgsql/data/postmaster.pid`
    <productname>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.  Other Unix-like systems may also work but are not currently
    being tested.  In most cases, all CPU architectures supported by
    a given operating system will work.  Look in