Typo: destuction --> destruction

Reported by Thomas Heller.

Doc/lib/libweakref.tex

@@ -248,6 +248,6 @@ instance_dealloc(PyInstanceObject *inst)
 
     PyObject_ClearWeakRefs((PyObject *) inst);
 
-    /* Proceed with object destuction normally. */
+    /* Proceed with object destruction normally. */
 }
 \end{verbatim}