Fix typo for new exercise part referencing wrong context

Author: DavidDiez

ch_intro_to_data/TeX/case_study_using_stents_to_prevent_strokes.tex

@@ -91,10 +91,15 @@
 control group?
 \item In which group did a higher percentage of patients experience improvement
 in symptoms?
-\item Your findings so far might suggest that a real difference in effectiveness of 
-antibiotic and placebo treatments for improving symptoms of sinusitis. However this is 
-not the only possible conclusion that can be drawn based on your findings so far. What is 
-one other possible explanation for the observed difference between the percentages 
-of patients that are pain free 24 hours after receiving acupuncture in the two groups?
+\item
+    Your findings so far might suggest a real difference
+    in effectiveness of antibiotic and placebo treatments
+    for improving symptoms of sinusitis.
+    However, this is not the only possible conclusion that
+    can be drawn based on your findings so far.
+    What is one other possible explanation for the observed
+    difference between the percentages of patients in the
+    antibiotic and placebo treatment groups that experience
+    improvement in symptoms of sinusitis?
 \end{parts}
 }{}