fix: typo '--aac' not '-aac'

usage example has 'aac' prefixed with single dash '-' (but should be double dashes '--')

README.md

@@ -25,7 +25,7 @@ Audible fails for some reason.
 
 ## Usage(s)
 ```
-bash AAXtoMP3 [-f|--flac] [-o|--opus] [-a|-aac] [-s|--single] [--level <COMPRESSIONLEVEL>] [-c|--chaptered] [-e:mp3] [-e:m4a] [-e:m4b] [-A|--authcode <AUTHCODE>] [-n|--no-clobber] [-t|--target_dir <PATH>] [-C|--complete_dir <PATH>] [-V|--validate] [--use-audible-cli-data]] [-d|--debug] [-h|--help] [--continue <CHAPTERNUMBER>] <AAX/AAXC INPUT_FILES>...
+bash AAXtoMP3 [-f|--flac] [-o|--opus] [-a|--aac] [-s|--single] [--level <COMPRESSIONLEVEL>] [-c|--chaptered] [-e:mp3] [-e:m4a] [-e:m4b] [-A|--authcode <AUTHCODE>] [-n|--no-clobber] [-t|--target_dir <PATH>] [-C|--complete_dir <PATH>] [-V|--validate] [--use-audible-cli-data]] [-d|--debug] [-h|--help] [--continue <CHAPTERNUMBER>] <AAX/AAXC INPUT_FILES>...
 ```
 or if you want to get guided through the options
 ```