Update syntax image examples to use images path

[kilfoyle]

As a convention our images are stored in directories named images, but our syntax instructions use img in the example image paths. This caused confusion for one developer so far, so I think it's worth updating our guide.

[colleenmcginnis]

@kilfoyle I think you'll also need to rename the docs/syntax/img directory to docs/syntax/images and then update any other references to images outside this Markdown file to also point to images instead of img.

[kilfoyle]

Thanks @colleenmcginnis! I renamed docs/syntax/img to docs/syntax/images as you suggested, and have fixed up the references to the images in that renamed directory.