Tyops in release announcement

Spotted to two typos in the 9.6.2 etc release announcement (a bit late to the
party I know).  No patch attached as these are in the database under id 1733.

s/These/This/ in the following:
   "These release contains several fixes to improve the stability of visible   data and WAL logging that we wish to highlight here."

s/Theses/These/ in the following:
   "Theses issues are present in the 9.6 series of PostgreSQL releases and may   also be present in the 9.2, 9.3, 9.4, and 9.5 series.”

cheers ./daniel