Add missing name substitution in __Pyx_PyUnicode_Substring() utility code.

Cython/Utility/StringTools.c

@@ -545,6 +545,7 @@
             PyObject* text, Py_ssize_t start, Py_ssize_t stop);
 
 /////////////// PyUnicode_Substring ///////////////
+//@substitute: naming
 
 static CYTHON_INLINE PyObject* __Pyx_PyUnicode_Substring(
             PyObject* text, Py_ssize_t start, Py_ssize_t stop) {