Fix docstring for workflow.defn's dynamic argument

[scorphus]

What was changed

This CL fix the docstring for the dynamic argument of the workflow.defn function, replacing one occurrence of activity by workflow to better reflect the usage of such attribute. I assume that was leftover of some copy'n'paste.

Why?

workflow.defn is used to decorate a workflow class, not an activity.

Checklist

1. Closes ✅
   None

2. How was this tested: ✅
   No tests necessary, I guess.

5. Any docs updates needed? ✅
   All good here too.

Valeu pelo Temporal, turma! Ele faz miséria! <3

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[dandavison]

LGTM, thanks!