Fix str(WindowedValueCoder) crash when underlying coder isn't KV coder (#37406)

Author: Abacn

sdks/python/apache_beam/coders/coders.py

@@ -1575,7 +1575,7 @@ def value_coder(self):
   def __repr__(self):
     return (
         f'WindowedValueCoder[window_coder={self.window_coder}, '
-        f'value_coder={self.value_coder()}]')
+        f'wrapped_value_coder={self.wrapped_value_coder}]')
 
   def __eq__(self, other):
     return (