changed default value to float (#4366)

* changed default value to float

* added type hint of float to target_soc

src/pybamm/parameters/parameter_values.py

@@ -79,7 +79,7 @@ def __init__(self, values, chemistry=None):
                 pybamm.citations.register(citation)
 
     @staticmethod
-    def create_from_bpx(filename, target_soc=1):
+    def create_from_bpx(filename, target_soc: float = 1):
         """
         Parameters
         ----------