Fix error message when adding conflicting agent functions.

The error message was attempting to print the name of both the agent and
of the function being added, however it was accidentally using the
agent's name twice instead.

src/flamegpu/model/LayerDescription.cpp

@@ -134,7 +134,7 @@ void LayerDescription::addAgentFunction(const std::string &agentName, const std:
                             THROW exception::InvalidAgentFunc("Agent function '%s' owned by agent '%s' cannot be added to this layer as agent function '%s' "
                                 "within the layer shares an input or output state, this is not permitted, "
                                 "in LayerDescription::addAgentFunction().",
-                                a->second->name.c_str(), agentName.c_str(), b->name.c_str());
+                                f->second->name.c_str(), agentName.c_str(), b->name.c_str());
                         }
                     }
                     // Check that the layer does not already contain function for the agent + state being output to