Update advanced_usage page

[eyalroz]

This page:

https://github.com/embeddedartistry/arduino-printf/blob/master/advanced_usage.md

discusses the possibilities using mpaland/printf ; now, putchar_() has been renamed, but more importantly - fctprintf() and vfcptprintf() are available, allowing even more flexibility.

[phillipjohnston]

Thanks for pointing this out! fctprintf being available makes me think I can improve the arduino-logging library usage too.