pgsql: docs: remove second mention of btree max length reduction

docs: remove second mention of btree max length reduction

I already added that to the incompatibility section as a separate item.

Reported-by: Peter Geoghegan

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4217d15d91128355ea13e6fb9c031b826e2a1335

Modified Files
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doc/src/sgml/release-12.sgml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)