Update materials.py (#1675)

Update materials.py

Author: Seb-Pi

src/ansys/mapdl/core/_commands/preproc/materials.py

@@ -341,7 +341,7 @@ def mpcopy(self, matf="", matt="", **kwargs):
 
         This command is also valid in SOLUTION.
         """
-        command = "MPCOPY,%s,%s" % (str(matf), str(matt))
+        command = "MPCOPY,,%s,%s" % (str(matf), str(matt))
         return self.run(command, **kwargs)
 
     def mpdata(