correct DOMDocument insertBefore call
correct DOMDocument insertBefore call

The current documentation showing how to insert an amp-ad unit before the 4th paragraph is incorrect. As previously written, the code would insert the ad code before the closing `<\p>` tag. Corrected so that the ad code gets inserted as a child of the body element.
ampproject/amp-toolbox-php,ampproject/amp-toolbox-php