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
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0e3215292f48f3e204402a07140322b63a30de36

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