fix markup errors icon examples and code

- `</span>` changed to`</div>` in `.icon-fallback-text` examples and code 
- in code tab, move mastercard to align with order in example tab
- in code tab, move `.icon-cart`; add `.icon-arrow-down` and `.icon-x` to align with order in example tab
- add `.feedback-text` to `.icon-arrow-down` in example tab
Merge pull request #133 from jholl/patch-12

Fix markup errors in icon examples and code