add missing ref

Author: TorkelE

docs/src/model_simulation/finite_state_projection_simulation.md

@@ -183,7 +183,7 @@ for endpoint in esol
 end
 heatmap(0:24, 0:24, ss_jump ./length(esol); xguide = "X₂", yguide = "X")
 ```
-Here we used an ensemble [output function]() to only save the final state of each simulation (and then plot these using `heatmap`).
+Here we used an ensemble [output function](@ref activation_time_distribution_measurement) to only save the final state of each simulation (and then plot these using `heatmap`).
 
 
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