Fix a couple of incorrect type annotations

h/t @justmobilize in #8891

Author: jepler

shared-bindings/synthio/LFO.c

@@ -62,7 +62,7 @@ STATIC const uint16_t triangle[] = {0, 32767, 0, -32767};
 //|
 //|     def __init__(
 //|         self,
-//|         waveform: ReadableBuffer = None,
+//|         waveform: Optional[ReadableBuffer] = None,
 //|         *,
 //|         rate: BlockInput = 1.0,
 //|         scale: BlockInput = 1.0,

shared-bindings/synthio/Note.c

@@ -65,7 +65,7 @@ static const mp_arg_t note_properties[] = {
 //|         filter: Optional[Biquad] = None,
 //|         ring_frequency: float = 0.0,
 //|         ring_bend: float = 0.0,
-//|         ring_waveform: Optional[ReadableBuffer] = 0.0,
+//|         ring_waveform: Optional[ReadableBuffer] = None,
 //|         ring_waveform_loop_start: int = 0,
 //|         ring_waveform_loop_end: int = waveform_max_length,
 //|     ) -> None: