update run commands to use correct identifier

[ma-pe]

It seems to me that, at least in version 4.0.0, the command name change to generator.

I adjusted this in the README.md.

In addition to that, when activating the package globally, I had to call generator directly instead of icon_font_generator:generator.

[ma-pe]

Just realised that this fix is also part of #33 and of #35.

It is more thorough than #33 and less invasive than #35. That's why I'll leave it open for the author to decide which one will be merged.