[DOCS] Mispellings in 9.0 docs (and probably lower too)

[Thom Brown]

http://www.postgresql.org/docs/9.0/static/different-replication-solutions.html

s/propogated/propagated/

http://www.postgresql.org/docs/9.0/static/hot-standby.html

s/shapshot/snapshot/

s/statististics/statistics/

Regards

Thom