Fix typo

Doc/api/newtypes.tex

@@ -441,7 +441,7 @@ Foo_Type.ob_type = &PyType_Type;
 
 \begin{cmemberdesc}{PyTypeObject}{int}{tp_basicsize}
 \cmemberline{PyTypeObject}{int}{tp_itemsize}
-  These fields allow calculating the size in byte of instances of
+  These fields allow calculating the size in bytes of instances of
   the type.
 
   There are two kinds of types: types with fixed-length instances have