Remove parens around single argument in React example

Parens around a single argument aren't in line with the js guide:

    If your function takes a single argument and doesn’t use braces,
    omit the parentheses.

https://github.com/airbnb/javascript/blob/0814be638f22b01a8e49efd77094be24055a3d48/README.md#arrows--one-arg-parens

Another rule violation:

    Wrap JSX tags in parentheses when they span more than one line.

https://github.com/airbnb/javascript/blob/0814be638f22b01a8e49efd77094be24055a3d48/react/README.md#parentheses