Added amp-iframe title attribute & modify indentation (#961)
Added amp-iframe title attribute & modify indentation (#961)

1. Added`title` attribute to this `<amp-iframe>` example to describe the content of this iframe. In this example, "Performs a live query against the AMP URL API"

[W3C H64: Using the title attribute of the frame and iframe elements:](https://www.w3.org/TR/WCAG20-TECHS/H64.html)
"title attribute of the frame or iframe element to describe the contents of each frame. This provides a label for the frame so users can determine which frame to enter and explore in detail. It does not label the individual page (frame) or inline frame (iframe) in the frameset."

[WCAG 2.4.1 Bypass Blocks:](https://www.w3.org/TR/2016/NOTE-UNDERSTANDING-WCAG20-20161007/navigation-mechanisms-skip.html) A mechanism is available to bypass blocks of content that are repeated on multiple Web pages. (Level A)

2. Modified indentation for `<amp-img>`
ampproject/amp-by-example,ampproject/amp-by-example,ampproject/amp-by-example,ampproject/amp-by-example