Removed remaining references to 'example_materials'.

Home screen now looks something like this:

    Abjad IDE

        example scores:
           1: blue example score
           2: etude example score
           3: red example score
        example stylesheets:
           4: clean-letter-14
           5: clean-letter-16
           6: rhythm-letter-16
           7: time-signature-context
        library: makers:
           8: zagged pitch class maker
        library: materials:
           9: baca numbers
          10: talea rhythm maker
          11: zagged pitch classes
        library: stylesheets:
          12: baca-letter-14
        scores:
          13: aracilik
          14: archipel
          15: betoerung
          16: cary
          17: chrysanthemums
          18: desir
          19: gebiete
          20: imaginaire
          21: krummzeit
          22: lidercfeny
          23: mannigfaltigkeiten
          24: manos
          25: recursif
          26: red shift
          27: sekka
          28: tack
          29: territoires
          30: traiettorie
          31: zeit

    >