spec: fix typo (#616)

Signed-off-by: Timothy Gu <[email protected]>
diff --git a/doc/spec.md b/doc/spec.md
index c5c1e71..29a38cb 100644
--- a/doc/spec.md
+++ b/doc/spec.md
@@ -3466,7 +3466,7 @@
 x.setdefault("three", 0)                # 0
 x                                       # {"one": 1, "two": 2, "three": 0}
 x.setdefault("four")                    # None
-x                                       # {"one": 1, "two": 2, "three": None}
+x                                       # {"one": 1, "two": 2, "three": 0, "four": None}
 ```
 
 <a id='dict·update'></a>