fixed typo

Objects/unicodeobject.c

@@ -233,7 +233,7 @@ PyUnicodeObject *_PyUnicode_New(Py_ssize_t length)
 {
     register PyUnicodeObject *unicode;
 
-    /* Optimization fo empty strings */
+    /* Optimization for empty strings */
     if (length == 0 && unicode_empty != NULL) {
         Py_INCREF(unicode_empty);
         return unicode_empty;