"ENTER" rather than "Enter" in check for keyup value in tutorial example on README.md

[mdmeacham]

Describe the bug
On README.md on https://github.com/centrifugal/centrifuge, the example index.html has a check for which key is used in the input element.

The current javascript has if (e.key === "ENTER") with uppercase ENTER

Should be if (e.key === "Enter")

Causes example index.html to not react to user input

To Reproduce
Steps to reproduce the behavior:

1. Follow tutorial and see that user input in browser does not send message

Expected behavior
Pressing enter on input element causes message to be sent

[FZambia]

Hello @mdmeacham

Many thanks for the report - just fixed. I guess there were some people who stopped looking at the library after the readme example failure 🫤