Fix error message

Fix #1831

Author: YingboMa

src/systems/diffeqs/odesystem.jl

@@ -316,7 +316,7 @@ function build_explicit_observed_function(sys, ts;
                     subs[s] = s′
                     continue
                 end
-                throw(ArgumentError("$s is either an observed nor a state variable."))
+                throw(ArgumentError("$s is neither an observed nor a state variable."))
             end
             continue
         end