Fix solution to exercise 3 of Functional Programming (#313)

* fix solution of exercise 3

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* Fix type hints

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Edoardo Baldi <edoardo.baldi@empa.ch>

Author: 3 people

tutorial/tests/test_11_functional_programming.py

@@ -142,7 +142,7 @@ def reference_multiples_of_n(my_list: list[int], k: int) -> list[int]:
     ],
 )
 def test_multiples_of_n(
-    function_to_test: Callable[[list[int]], int],
+    function_to_test: Callable[[list[int], int], int],
     my_list: list[int],
     k: int,
 ):
@@ -163,7 +163,8 @@ def reference_exercise1(matrix: list[list[int]]) -> list[list[int]]:
     [(np.eye(3)), (np.random.randint(0, 100, size=(4, 4)))],
 )
 def test_exercise1(
-    function_to_test: Callable[[list[list[int]]], list[list[int]]], my_input: NDArray
+    function_to_test: Callable[[list[list[int]]], list[list[int]]],
+    my_input: NDArray,
 ):
     res = function_to_test(my_input.tolist())
     assert (
@@ -210,12 +211,14 @@ def reference_exercise3(words: list[str]) -> list[tuple[str, int]]:
     return [
         (k, len(list(v)))
         for k, v in itertools.groupby(
-            sorted(words, key=lambda x: x[0]), key=lambda x: x[0]
+            sorted(words, key=lambda x: x.lower()[0]), key=lambda x: x.lower()[0]
         )
     ]
 
 
-def test_exercise3(function_to_test: Callable[[list[str]], list[tuple[str, int]]]):
+def test_exercise3(
+    function_to_test: Callable[[list[str]], list[tuple[str, int]]],
+):
     data = get_data_exercise3()
     assert function_to_test(data) == reference_exercise3(data)
 
@@ -225,7 +228,9 @@ def test_exercise3(function_to_test: Callable[[list[str]], list[tuple[str, int]]
 #
 
 
-def reference_exercise4(my_list: list[tuple[str, int]]) -> list[tuple[str, float]]:
+def reference_exercise4(
+    my_list: list[tuple[str, int]],
+) -> list[tuple[str, float]]:
     total = sum(map(lambda x: x[1], my_list))  # noqa: C417
     return [(letter, freq / total) for letter, freq in my_list]