autotest: correct diagnostic string

that "f" in front of an f-string is important

Tools/autotest/vehicle_test_suite.py

@@ -5279,7 +5279,7 @@ def mission_item_protocol_items_from_filepath(self,
                                                   ):
         '''returns a list of mission-item-ints from filepath'''
         # self.progress("filepath: %s" % filepath)
-        self.progress("Loading {loaderclass.itemstype()} (%s)" % os.path.basename(filepath))
+        self.progress(f"Loading {loaderclass.itemstype()} (%s)" % os.path.basename(filepath))
         wploader = loaderclass(
             target_system=target_system,
             target_component=target_component