Slider Example with aria-orientation: Editorial Revisions

For issue #230, made following changes to  examples/slider/slider-2.html to make it  consistent with editorial guidelines and the template for examples:

1. Changed title to "Slider Examples with aria-orientation and aria-valuetext"
2. Made H1 consistent with title element.
3. Added link to review issue.
4. Added link to design pattern in intro paragraph and made editorial revisions and corrections.
5. Made editorial revisions and corrections to horizontal slider link and description.
6. Removed region role from example container.
7. Removed an H2 heading before the horizontal sliders and made all three sliders part of a single example container.
8. Added missing separator elements that mark the start and end of the example container for screen reader users.
9. Made editorial revisions to the keyboard table.
10. Made text of H2 for roles, states, and properties consistent with template.
11. Added a row for tabindex to the attributes table and made other editorial revisions and corrections.
12. Fixed href value of design pattern link in nav footer.