Update docs/src/model_creation/dsl_advanced.md

Author: isaacsas

docs/src/model_creation/dsl_advanced.md

@@ -283,7 +283,9 @@ rn = @reaction_network begin
 end
 ```
 Running the code above will yield the following error: 
-
+```
+LoadError: UndeclaredSymbolicError: Unrecognized variables A detected in reaction expression: "((k1, k2), A <--> B)". Since the flag @require_declaration is declared, all species must be explicitly declared with the @species macro.
+```
 In order to avoid the error in this case all the relevant species and parameters will have to be declared.
 ```@example dsl_advanced_require_dec
 # The following case will not error.