Update invalid pandas indexing statement (#580)

Author: phofl

episodes/03-index-slice-subset.md

@@ -390,7 +390,7 @@ selects the element that is 3 rows down and 7 columns over in the DataFrame.
 It is worth noting that rows are selected when using `loc` with a single list of
 labels (or `iloc` with a single list of integers). However, unlike `loc` or `iloc`,
 indexing a data frame directly with labels will select columns (e.g.
-`surveys_df['species_id', 'plot_id', 'weight']`), while ranges of integers will
+`surveys_df[['species_id', 'plot_id', 'weight']]`), while ranges of integers will
 select rows (e.g. surveys\_df[0:13]). Direct indexing of rows is redundant with
 using `iloc`, and will raise a `KeyError` if a single integer or list is used; the
 error will also occur if index labels are used without `loc` (or column labels used