Fixing a typo in README.md

[koksyn]

For "Set up" section in documentation, there was a typo in import example for ES6.

"import Big from 'big.jms'"
instead of
"import Big from 'big.js'"

[MikeMcl]

The existing file extension is correct: big.mjs is the ES6 module version of big.js.

See, for example, the Node.js documentation:

The --experimental-modules flag can be used to enable features for loading ESM modules. Once this has been set, files ending with .mjs will be able to be loaded as ES Modules.

However, you are correct that there is a mistake - for the import to work in Node.js and browsers, it should be a relative path

import Big from './big.mjs';

If you update this PR accordingly, I will accept it.

[koksyn]

Sorry for the confusion with the file extension. I didn't even know before that there are such experimental modules.

I restored the ".mjs" and changed the path to relative as you suggested.