Fix missing f-string format for bit fields (#59)

dissect/cstruct/types/structure.py

@@ -22,7 +22,7 @@ def __init__(self, name: str, type_: BaseType, bits: int = None, offset: int = N
         self.alignment = type_.alignment
 
     def __repr__(self):
-        bits_str = " : {self.bits}" if self.bits else ""
+        bits_str = f" : {self.bits}" if self.bits else ""
         return f"<Field {self.name} {self.type}{bits_str}>"