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: https://postgr.es/m/DC9A662A-360D-4125-A453-5A6CB9C6C4B4@yesql.se
Backpatch-through: v11

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/385b317715179fd60b6a2899a3d10b13ca52ad4d

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