pgsql: Replace link to Hunspell with the current homepage

Replace link to Hunspell with the current homepage

The Hunspell project moved from Sourceforge to Github sometime
in 2016, so update our links to match the new URL. Backpatch
the doc changes to all supported versions.

Discussion: /messages/by-id/DC9A662A-360D-4125-A453-5A6CB9C6C4B4@yesql.se
Backpatch-through: v11

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/4c3cb6b762061645cfa7efea16c159ee0cbc78bf

Modified Files
--------------
doc/src/sgml/textsearch.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)