Fix incorrect variable name bug

Fix misleading variable name.  `$author` -> `$artist`

This took 30 mins of head pounding to figure out what was wrong :-)

AAXtoMP3

@@ -16,7 +16,7 @@ level=-1                    # Compression level. Can be given for mp3, flac and
 mode=chaptered              # Multi file output
 auth_code=                  # Required to be set via file or option.
 targetdir=                  # Optional output location.  Note default is basedir of AAX file.
-dirNameScheme=              # Custom directory naming scheme, default is $genre/$author/$title
+dirNameScheme=              # Custom directory naming scheme, default is $genre/$artist/$title
 customDNS=0
 fileNameScheme=             # Custom file naming scheme, default is $title
 customFNS=0