Documentation for `awful.screenshot`'s property `file_name` is wrong

[simplepad]

Documentation states that the file_name's default value is: https://github.com/awesomeWM/awesome/blob/b54e50ad6cfdcd864a21970b31378f7c64adf3f4/lib/awful/screenshot.lua#L415

However, actually the file_name is generated like this: https://github.com/awesomeWM/awesome/blob/b54e50ad6cfdcd864a21970b31378f7c64adf3f4/lib/awful/screenshot.lua#L156

So the documentation misses the "_" between self.prefix and date_time.