Update documentation for Electron 5
Update documentation for Electron 5

The documentation for `webFrame.setSpellCheckProvider` does not require a 'true' flag in the second position, anymore.
####### webFrame.setSpellCheckProvider('en-US', **true**, new SpellCheckProvider('en-US'))
https://electronjs.org/docs/api/web-frame#webframesetspellcheckproviderlanguage-provider

This throws an error in Electron version 5 and higher. I have not tried this with previous versions.
mixmaxhq/electron-spell-check-provider