Expand multiple sdr wiki

[duk3luk3]

Wiki: https://github.com/projecthorus/radiosonde_auto_rx/wiki/Configuration-Settings#use-of-multiple-rtlsdrs

Suggested changes:

• Change serial example to a non-numeric string e.g. N0000002 to document that serial numbers can be non-numeric and avoid the mentioned issues with rtl_ commands interpreting numeric serial numbers as device index
• Add a note that rtl_ commands accept serial numbers for the -d parameter as this is not obvious and not documented in manpages

[darksidelemm]

Have fixed the first point. Not sure where in particular you want me to mention the use of -d