Fix typo

Author: gjbex

source-code/functional-programming/README.md

@@ -9,4 +9,4 @@ Some examples of a functional programming style in Python.
 1. `persistent_lists.ipynb`: example of an implementation of a persistent
    list, i.e., a purely functional data structure.
 1. `persistent_binary_search_tree.ipynb`: example of an implementation of a
-   persistent list, i.e., a purely functional data structure.
+   persistent binary search tree, i.e., a purely functional data structure.