Correcting "make html" errors

@mhoeber @srpearce I made an html version of the course-authors guide this morning and got some errors. 
                                
.../course_authors/source/exercises_tools/problem_with_hint.rst:64: WARNING: duplicate label drag and drop problem xml, other instance in .../course_authors/source/exercises_tools/drag_and_drop.rst
.../course_authors/source/exercises_tools/vitalsource.rst:5: WARNING: Duplicate explicit target name: "vitalsource bookshelf support site".
.../course_authors/source/releasing_course/beta_testing.rst:198: SEVERE: Title level inconsistent: (2 of these)

I made these changes to correct them.