[hotfix][docs] wrong method name in state-processor-api example

[alpinegizmo]

What is the purpose of the change

The State Processor API docs have an example of implementing a KeyedStateReaderFunction. The wrong method name is used in this example -- the class has no processKey method, but rather a readKey method.

Verifying this change

This change is a trivial typo fix.

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Last check on commit b30983f (Mon Aug 26 08:25:25 UTC 2019)

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CI report:

• b30983f : SUCCESS Build

[1u0]

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[StephanEwen]

Thank you for the fixing this!

Looks good, merging...