Lint example references by name

[peterbe]

If you look at https://raw.githubusercontent.com/mdn/stumptown-content/master/content/html/reference/elements/input/input.md there's this:

examples:
    - examples/examples_overview
    - examples/Styling_input_elements
    - examples/Custom_error_messages

But one of those don't exist. There is examples/examples_overview. There's a examples/Examples_overview though. Easy mistake to make. Let's prevent it from happening again.