[FLINK-12339][example] Fix variable name

[mjl18758265781]

What is the purpose of the change

This pull request rename topSeed to topSpeed in TopSpeedWindowing.scala. This variable is correct in this TopSpeedWindowing.java

Verifying this change

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• 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, Yarn/Mesos, 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)

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