Fix punctuation.
diff --git a/Doc/library/weakref.rst b/Doc/library/weakref.rst
index 1c35493..d23e53a 100644
--- a/Doc/library/weakref.rst
+++ b/Doc/library/weakref.rst
@@ -77,8 +77,8 @@
obj = Dict(red=1, green=2, blue=3) # this object is weak referenceable
Other built-in types such as :class:`tuple` and :class:`long` do not support
-weak references even when subclassed (This is an implementation detail and may
-be different across various Python implementations.).
+weak references even when subclassed (this is an implementation detail and may
+be different across various Python implementations).
Extension types can easily be made to support weak references; see
:ref:`weakref-support`.