Change backslashes in link tag in elements.html

In the file `app\elements\elements.html`:
`<link rel="import" href="my-element\my-element.html">`
should be 
`<link rel="import" href="my-element/my-element.html">`


The elements.html file, has a link tag with an href attribute.  The
value of href on Windows is rendered with backslashes instead of forward
slashes. Changed.