correct readme for md.eco example

To use marked markdown with eco, text plugin example needs html.md.eco

As the example is listed, the `md.eco` example will not render the
markdown, but instead `**0.7382947329847923**`

However if you simply add `html` prefix then it will process it properly. `html.md.eco`

This may work differently with other markdown processors, but it should be safe to
include anyway.