Fix typo on MutableSequence docstring.
diff --git a/Lib/collections/abc.py b/Lib/collections/abc.py
index 8bac957..a8681ea 100644
--- a/Lib/collections/abc.py
+++ b/Lib/collections/abc.py
@@ -662,7 +662,7 @@
 
     __slots__ = ()
 
-    """All the operations on a read-only sequence.
+    """All the operations on a read-write sequence.
 
     Concrete subclasses must provide __new__ or __init__,
     __getitem__, __setitem__, __delitem__, __len__, and insert().