Fix the code in the example in the README and add a test to verify this

Makefile

@@ -1,9 +1,7 @@
 .PHONY: docs
 
 flake:
-	flake8 evol
-	flake8 tests
-	flake8 setup.py
+	python setup.py flake8
 
 install:
 	pip install -e .

README.md

@@ -108,13 +108,13 @@ pop = Population(chromosomes=[random_start() for _ in range(200)],
 evo1 = (Evolution()
        .survive(fraction=0.5)
        .breed(parent_picker=pick_random_parents, combiner=make_child)
-       .mutate(func=add_noise, sigma=1))
+       .mutate(mutate_function=add_noise, sigma=1))
 
 # We define another sequence of steps to change these candidates
 evo2 = (Evolution()
        .survive(n=1)
        .breed(parent_picker=pick_random_parents, combiner=make_child)
-       .mutate(func=add_noise, sigma=0.2))
+       .mutate(mutate_function=add_noise, sigma=0.2))
 
 # We are combining two evolutions into a third one. You don't have to
 # but this approach demonstrates the flexibility of the library.

tests/test_readme.py

@@ -0,0 +1,20 @@
+from pytest import fixture
+
+
+@fixture(scope='function')
+def readme_code() -> str:
+    content = []
+    in_code_block = False
+    with open('../README.md') as readme_file:
+        for line in readme_file.readlines():
+            if in_code_block:
+                if line == '```\n':
+                    return ''.join(content)
+                else:
+                    content.append(line)
+            elif line == '```python\n':
+                in_code_block = True
+
+
+def test_readme(readme_code):
+    exec(readme_code, globals())