03-index-slice-subset.md: Fix variable name (#487)

Change `ref_survey_df` to `ref_surveys_df`

Author: Jarrod

_episodes/03-index-slice-subset.md

@@ -249,8 +249,8 @@ What is the difference between these two dataframes?
 
 When we assigned the first 3 columns the value of `0` using the
 `ref_surveys_df` DataFrame, the `surveys_df` DataFrame is modified too.
-Remember we created the reference `ref_survey_df` object above when we did
-`ref_survey_df = surveys_df`. Remember `surveys_df` and `ref_surveys_df`
+Remember we created the reference `ref_surveys_df` object above when we did
+`ref_surveys_df = surveys_df`. Remember `surveys_df` and `ref_surveys_df`
 refer to the same exact DataFrame object. If either one changes the object,
 the other will see the same changes to the reference object.