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
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/926ba562ef4f85e922aef6ee64a5209fe2d00d54

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