Fix some bugs

Change lint.py to print the name of file being linted for easy debugging. Also removed a bug from website\main.py

snippets/difference.md

@@ -6,7 +6,7 @@ Use list comprehension to only keep values not contained in `b`
 
 ```python
 def difference(a, b):
-    return [item for item in a if item not in b]
+    return [item for item in a if item not in b]
 ```
 ``` python
 difference([1, 2, 3], [1, 2, 4]) # [3]