deprecated `navigator.epubReadingSystem.name` used in example

[npdoty]

I believe navigator.epubReadingSystem.name has been deprecated, and replaced with feature detection, but there is still an example that uses it in "H.2 Scripting contexts".

[mattgarrish]

The script element with the reference can be removed without affecting anything about the example itself. Not sure why it was put in there.

[iherman]

I think having the script element kept there is better; good to have an example for such a construction just to show. I simply exchanged the usage of the name attribute to hasFeature and slightly reformulated the script content.

Thanks @npdoty. See #2544