Fix typo.

Doc/api/concrete.tex

@@ -2536,7 +2536,7 @@ when accessed.  Cell objects are not likely to be useful elsewhere.
 
 \begin{cfuncdesc}{PyObject*}{PyCell_GET}{PyObject *cell}
   Return the contents of the cell \var{cell}, but without checking
-  that \var{cell} is non-\NULL{} and a call object.
+  that \var{cell} is non-\NULL{} and a cell object.
 \end{cfuncdesc}
 
 \begin{cfuncdesc}{int}{PyCell_Set}{PyObject *cell, PyObject *value}