_platform/darwin: fix missing space, say MacOS
- Fix a missing space between words in error message
- Say MacOS instead of OSX
Note that 'OSX' is now called 'MacOS', as of 2016: https://en.wikipedia.org/wiki/MacOS
Note that 'OSX' is now called 'MacOS', as of 2016: https://en.wikipedia.org/wiki/MacOS