message: Fix spelling in progress demo
diff: <h4>Loading </h4>
  -<h5>Progressbar</h5>
  +<h5>Progress Bar</h5>
  -<h6>Simple Porgressbar</h6>
  +<h6>Simple Progress Bar</h6>
  <mdl-progress [progress]="progress"></mdl-progress>
  Progress:
  <input type="number" min="0" max="100" [(ngModel)]="progress">
  