Udpate readme.md / Grammar fix (comma mistake)

[jonisar]

I've deleted the comma in the sentence:
"JSX is not required to use React, but it makes code more readable, and writing it feels like writing HTML."

When the sentence only contains two listed items, they should not be separated by a comma before using "and".
You should not use a comma before and if you’re only mentioning two qualities.

The dog is well trained and good natured. - V Right way to use
The dog is well trained, and good natured. - X Wrong way to use

See: https://www.grammarly.com/blog/comma-before-and/

Hence:

...it makes code more readable and writing it feels like writing HTML. - V
...it makes code more readable, and writing it feels like writing HTML. -X

Cheers :) Keep up the amazing work!

Before submitting a pull request, please make sure the following is done:

1. Fork the repository and create your branch from master.
2. Run yarn in the repository root.
3. If you've fixed a bug or added code that should be tested, add tests!
4. Ensure the test suite passes (yarn test). Tip: yarn test --watch TestName is helpful in development.
5. Run yarn test-prod to test in the production environment. It supports the same options as yarn test.
6. If you need a debugger, run yarn debug-test --watch TestName, open chrome://inspect, and press "Inspect".
7. Format your code with prettier (yarn prettier).
8. Make sure your code lints (yarn lint). Tip: yarn linc to only check changed files.
9. Run the Flow typechecks (yarn flow).
10. If you haven't already, complete the CLA.

Learn more about contributing: https://reactjs.org/docs/how-to-contribute.html

[sizebot]

Details of bundled changes.

Comparing: 4947fcd...0e51626

scheduler

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
scheduler.development.js	n/a	n/a	0 B	19.17 KB	0 B	5.74 KB	UMD_DEV
scheduler.production.min.js	n/a	n/a	0 B	3.16 KB	0 B	1.53 KB	UMD_PROD

Generated by 🚫 dangerJS

[jonisar]

None- I deleted a redundant comma according to correct grammar rules :)

[sophiebits]

This is a compound sentence, so it deserves a comma – just like the one you're reading.