Statistics Bring It Together fix for practice section content duplication

In the Bring it Together sections of Statistics for the Practice sections, there is an XSLT issue that prevents the use of the same string in separate classes.  Due to this, content from class .bring-together-practice was being duplicated in the .practice class section above.  

Class name was changed to bring-together-exercises to alleviate this content duplication.  The other section of homework is safe, because the class name for homework is ".free-reponse", so .bring-together-homework doesn't cause the XSLT problem.