Update system.py (#524)

The example is wrong? "driver" should be "Driver"?

Signed-off-by: Levi Zhou  <31941107+ZhouXY-PKU@users.noreply.github.com>

Author: ZhouXY-PKU

dpdata/system.py

@@ -1410,7 +1410,7 @@ def minimize(
 
         >>> from dpdata.driver import Driver
         >>> from ase.optimize import BFGS
-        >>> driver = driver.get_driver("dp")("some_model.pb")
+        >>> driver = Driver.get_driver("dp")("some_model.pb")
         >>> some_system.minimize(minimizer="ase", driver=driver, optimizer=BFGS, fmax=1e-5)
         """
         if not isinstance(minimizer, Minimizer):