Update book docs to use Ruby::str_new

[ryanseys]

Update book docs to use new magnus Ruby::str_new instead of RString::new (which is now on old-api magnus feature)

Also shortened from "Hello Ruby!" to just "Ruby!" and reformatted alignment of comments to save some space on screen.

OLD source: https://github.com/matsadler/magnus/blob/b83762a95462fa87a1fae445f243e7eb9ae5c8ab/src/r_string.rs#L313-L340

NEW source: https://github.com/matsadler/magnus/blob/b83762a95462fa87a1fae445f243e7eb9ae5c8ab/src/r_string.rs#L46-L69