[ModelicaSystemCmd] spelling fix

Author: syntron

OMPython/ModelicaSystem.py

@@ -132,7 +132,7 @@ def __init__(self, runpath: pathlib.Path, modelname: str, timeout: Optional[int]
 
     def arg_set(self, key: str, val: str | dict = None) -> None:
         """
-        Set one argument for the executeable model.
+        Set one argument for the executable model.
 
         Parameters
         ----------