[FLINK-37383][flink-examples]Correct throttling logic on ThrottledIterator

[rafaelzimmermann]

What is the purpose of the change

The throttle function was updating its last batch check time before the sleep operation, causing it to underestimate the elapsed time and allow approximately double the intended throughput rate.

Brief change log

Moving the timestamp update to after the sleep ensures the elapsed time calculation properly accounts for the full duration between batches, maintaining the configured rate limit.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[flinkbot]

CI report:

• 600c9c2 UNKNOWN

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