fix error test

Author: valentinsulzer

tests/unit/test_solvers/test_base_solver.py

@@ -70,7 +70,7 @@ def test_nonmonotonic_teval(self):
         # Check stepping with step size too small
         dt = 1e-9
         with self.assertRaisesRegex(
-            pybamm.SolverError, "Step time must be at least 1.000 us"
+            pybamm.SolverError, "Step time must be at least 1.000 ns"
         ):
             solver.step(None, model, dt)