Fix a small error in an example of the README
Fix a small error in an example of the README

According to the [acorn-jsx](https://github.com/RReverser/acorn-jsx) documentation, the module returns a function that must be called when extending the parser. :)
TimothyGu/acorn,ternjs/acorn,mysticatea/acorn,ternjs/acorn,TimothyGu/acorn,ternjs/acorn,TimothyGu/acorn,mysticatea/acorn,marijnh/acorn,marijnh/acorn,marijnh/acorn,mysticatea/acorn