[typo] fix dupe in datetime.fromisoformat docs (GH-17295)

[typo] fix dupe in datetime.fromisoformat docs (GH-17295)

Fixes a nearly word for word duplication of a sentence that appears
earlier in the caution section of datetime.datetime.fromisoformat in
Doc/Library/datetime.rst.

No issue created as it's a trivial change.

Automerge-Triggered-By: @pganssle
(cherry picked from commit e7b406f4e20e248d24079f97d7c8b93e5a45249e)

Co-authored-by: Michael Morehouse <640167+yawpitch@users.noreply.github.com>