Reference correct DataFrame in to_pickle

The DataFrame without the changes made to it was passed `to_pickle`, causing the user to save the wrong data in the file.

Author: vinegar-tom

episodes/tidy.md

@@ -334,7 +334,7 @@ If we look at the data again, we will see our index will be set to date.
 Let's save `df_long` to use in the next episode. 
 
 ```python
-df.to_pickle('data/df_long.pkl')
+df_long.to_pickle('data/df_long.pkl')
 ```
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