doc: Fix description of configure --with-icu option

doc: Fix description of configure --with-icu option

It was claiming that the ICU locale provider is used by default, which
is not correct.  (From commit fcb21b3acdc; it was once contemplated to
make it the default, but it wouldn't have been part of that patch in
any case.)

Reviewed-by: Kashif Zeeshan <kashi.zeeshan@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/a71023c2-0ae0-45ad-9688-cf3b93d0d65b%40eisentraut.org