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MINOR: [Docs][Flight] Fix PollFlightInfo diagrams (#37499)

[Based on this note](https://lists.apache.org/thread/pr1xgrow1111k8h2n08phrbvk33n8d0w), it appears that RetryInfo was renamed to PollInfo, but the docs were not updated to reflect this renaming.  This commit changes references of RetryInfo to PollInfo.

CC: @ kou 

### Rationale for this change

Improve the documentation.

### What changes are included in this PR?

This is fixing a typo in the sequence diagrams of the documentation.

### Are these changes tested?

N/A

### Are there any user-facing changes?

No

Authored-by: Tim Meehan <tim@timdmeehan.com>
Signed-off-by: David Li <li.davidm96@gmail.com>