docs: correct step 1 of Output guardrails to use agent's output (#890)

[kirtiupacharya]

This PR fixes a typo in the “Output guardrails run in 3 steps” section by updating step 1 to refer to the agent’s output rather than its input.

• Changed “First, the guardrail receives the same input passed to the agent.”
  to “First, the guardrail receives the same output produced by the agent.”

Closes #890

[seratch]

Thank you for submitting this but we already have the fix for this issue (#903). So, let us close this one as duplicate 🙇