Fix typo in doc.go

[koho-volodymyr]

The use of square brackets around the workflow.ExecuteActivity invocation in the first example caused confusion for less experienced readers.

What was changed

Removed square brackets in the example used in doc.go

Why?

It's confusing for less experience readers.

Checklist

1. Closes

2. How was this tested:

3. Any docs updates needed?

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