Fix a typo in the PgTimestamp documentation
Fix a typo in the PgTimestamp documentation

[Relevant Postgres documentation](http://www.postgresql.org/docs/current/static/datatype-datetime.html)

tohou/diesel,mcasper/diesel,mcasper/diesel,tohou/diesel,diesel-rs/diesel,sgrif/diesel,diesel-rs/diesel,SoftwareWarlock/diesel,sgrif/diesel,cascalheira/diesel,cascalheira/diesel,SoftwareWarlock/diesel