Fix type annotation so it's ok in newest sphinx/autoapi

this otherwise gets
```
astroid.exceptions.InferenceError: Inference failed for all members of [<BinOp l.13 at 0x7fd379c99e80>].
```

Author: jepler

shared-bindings/synthio/__init__.c

@@ -34,7 +34,7 @@
 //|
 //|
 
-//| AnyBiquad = Biquad | BlockBiquad
+//| AnyBiquad = "Biquad | BlockBiquad"
 //|
 //| class EnvelopeState:
 //|     ATTACK: EnvelopeState