[HACKERS] Backport of pg_statistics typos fix

Hi,

I found typos "pg_statistics" in REL9_6_STABLE, but that has been
fixed in the master branch.

Fix typo: pg_statistics -> pg_statistic

https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=5a366b4ff4ceceb9793fcc13c3f097ee0d32c56d;hp=f7c62462402972b13d10e43f104ca0c0fecb6d08

I think it would be better to backport this to other branches.

Regards,

-- 
Yugo Nagata <nagata@sraoss.co.jp>