pgsql: doc: fix link to ALTER GROUP

doc:  fix link to ALTER GROUP

Fix for commit 2882d1f31a.

Reported-by: Tom Lane

Discussion: https://postgr.es/m/1388368.1696011440@sss.pgh.pa.us

Backpatch-through: 16 only

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/527c3c1bec65634135e2b1707ebc1a3c737f9458

Modified Files
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doc/src/sgml/release-16.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)