Fix code typo.

pep-0557.rst

@@ -212,8 +212,8 @@ Python syntax::
 
   @dataclass
   class C:
-      int a       # 'a' has no default value
-      int b = 0   # assign a default value for 'b'
+      a: int       # 'a' has no default value
+      b: int = 0   # assign a default value for 'b'
 
 For common and simple use cases, no other functionality is required.
 There are, however, some Data Class features that require additional