[Task][Go SDK]: Remove usage of anonymous function in doc example

[johannaojeling]

What needs to happen?

Remove the usage of an anonymous function in the fileio.ReadMatches doc example since anonymous functions are being cleaned up with the move to Prism as the default runner (see #29179).

Issue Priority

Priority: 2 (default / most normal work should be filed as P2)

Issue Components

• Component: Python SDK
• Component: Java SDK
• Component: Go SDK
• Component: Typescript SDK
• Component: IO connector
• Component: Beam YAML
• Component: Beam examples
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• Component: Twister2 Runner
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• Component: Google Cloud Dataflow Runner

[johannaojeling]

.take-issue