'prepositions' test in 05_book_titles/book_titles_spec.rb fails unnecessarily

[blerg-rush]

The test string for 'prepositions' fails even if the title method properly handles prepositions but not articles, because it contains the word "the". A better test for this case would only include prepositions that the method should leave uncapitalized; otherwise, students may be confused when this test fails.

A test for 'articles' already exists, so we don't need to test for both here.