Fix typo

numcodecs/bitround.py

@@ -47,7 +47,7 @@ def __init__(self, keepbits: [int, Callable]):
             raise ValueError("keepbits must be zero or positive")
 
         elif isinstance(keepbits, int):
-            self.keepbits = [lambda x: keepbits]
+            self.keepbits = lambda x: keepbits
 
         elif isinstance(keepbits, Callable):
             self.keepbits = keepbits