Reword the sentence for pg_logical_slot_peek_changes function.

Previously the duplicated paragraphs were used next to each other
in the document to demonstrate that the changes in the stream
were not consumed by pg_logical_slot_peek_changes function.
But some users misunderstood that the duplication of the same
paragraph was just typo. So this commit rewords the sentence in
the latter paragraph for less confusing.

Christoph Moench-Tegeder