fix ModifyPotcar doc string copy error

Author: janosh

atomate/vasp/firetasks/write_inputs.py

@@ -265,8 +265,8 @@ class ModifyPotcar(FiretaskBase):
 
     Optional params:
         functional (dict): functional to use, e.g. PBE, PBE_52, LDA_US, PW91
-        input_filename (str): Input filename (if not "INCAR")
-        output_filename (str): Output filename (if not "INCAR")
+        input_filename (str): Input filename (if not "POTCAR")
+        output_filename (str): Output filename (if not "POTCAR")
     """
 
     required_params = ["potcar_symbols"]