Fix typo explained in #509

[ryanfitzer]

Fix typo explained in #509.

[giuseppeg]

to be honest I don't know exactly what is the difference between filename and sourceFileName:

https://github.com/zeit/styled-jsx/blob/1b3497517ec8743db3f236fe3191482578da121b/src/babel.js#L230-L238

I asked on the Babel slack community, let's wait for a reply.

The part you changed could be correct and the bug could be here https://github.com/zeit/styled-jsx/blob/1b3497517ec8743db3f236fe3191482578da121b/src/babel-external.js#L205-L209 (which is not computing and passing filename

[giuseppeg]

@ryanfitzer do you mind taking a look at these and see if sourceFileName is really what we need? I won't have time tonight/in the following days (maybe on Wed)

https://babeljs.io/docs/en/options#filename
https://babeljs.io/docs/en/options#filenamerelative
https://babeljs.io/docs/en/options#sourcefilename

[ryanfitzer]

@lluisrojass If you get time, can you take a look at this before Wednesday? I won't have time until Monday.

[giuseppeg]

closed in favor of #526