Fix missing 'f' for format string

I encountered this error message during testing:

Traceback (most recent call last):
  File "/home/uli/dev/kicad-symbols/../kicad-library-utils/klc-check/comparelibs.py", line 156, in <module>
    new_lib = KicadLibrary.from_file(lib_path)
  File "/home/uli/dev/kicad-library-utils/common/kicad_sym.py", line 1211, in from_file
    raise KicadFileFormatError(
kicad_sym.KicadFileFormatError: Failed to parse subsymbol due to invalid name: {name}

The format string is missing a f at the front. This MR fixes that