Examples: Fix typo in HTML

[linbingquan]

No description provided.

[Mugen87]

There are three other example files with the same markup problem: software_sandbox.html, webgl_loader_nodes.html and webgl_performance_nodes.html. It would be good if you fix these too.

[linbingquan]

Thanks, here you are. 😄

[Mugen87]

Cool, you also thought about the docs! 😉

[mrdoob]

Thanks!