[DOCS] Back-and-forth link on GIN Indexes

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/9.6/static/gin-intro.html
Description:

Hello,

The page https://www.postgresql.org/docs/current/static/gin-intro.html
mentions "There is more information about GIN on their website." which
suggests to follow this link: http://www.sai.msu.su/~megera/wiki/Gin .

That page then redirects back to the postgreSQL documentation with the
following text:
"This is an original documentation and may be outdated. Actual version of
documentation is available from
http://www.postgresql.org/docs/current/static/gin-extensibility.html"

Methinks the first link (http://www.sai.msu.su/~megera/wiki/Gin) is
unnecessary.

Thanks,
Jean-Noel