update count_by to PEP8

snippets/count_by.md

@@ -10,7 +10,7 @@ Use `map()` to map the values of the list using the given function. Iterate over
 def count_by(arr, fn=lambda x: x):
     key = {}
     for el in map(fn, arr):
-        key[el] = 0 if not el in key else key[el]
+        key[el] = 0 if el not in key else key[el]
         key[el] += 1
     return key
 ```