Fixed various small errors in examples

[mrxz]

Description:
Tested some tooling I'm working on and found several small issues with the various examples in this repository. This PR addresses them.

Changes proposed:

• Removed the "flythrough" boilerplate. It referenced a non-existing image, used an invalid "thickness" property of <a-box> and wasn't listed in the main examples index.html
• Updated anime-UI to use the core hide-on-enter-ar component and (re)enabled AR button.
• Ensure <a-mixin> is always properly closed with an end tag (it isn't a void element)
• Fix various small typos, invalid values and delimiters

[dmarcos]

Thank you!