bpo-36321: Fix misspelled attribute in namedtuple() (GH-12375)

diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index daa7414..64de970 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -953,14 +953,14 @@
         >>> Pixel(11, 22, 128, 255, 0)
         Pixel(x=11, y=22, red=128, green=255, blue=0)
 
-.. attribute:: somenamedtuple._fields_defaults
+.. attribute:: somenamedtuple._field_defaults
 
    Dictionary mapping field names to default values.
 
    .. doctest::
 
         >>> Account = namedtuple('Account', ['type', 'balance'], defaults=[0])
-        >>> Account._fields_defaults
+        >>> Account._field_defaults
         {'balance': 0}
         >>> Account('premium')
         Account(type='premium', balance=0)