Fix pickle doc typo

Patch #1608758
(backport from r52917)

Doc/lib/libpickle.tex

@@ -519,7 +519,7 @@ as their value.  The semantics of each element are:
 version of the object.  The next element of the tuple will provide
 arguments for this callable, and later elements provide additional
 state information that will subsequently be used to fully reconstruct
-the pickled date.
+the pickled data.
 
 In the unpickling environment this object must be either a class, a
 callable registered as a ``safe constructor'' (see below), or it must