Fix usage output

[valeriyvan]

Currently if run with swift run script outputs:

Error: Missing expected argument '<count> ...'

OVERVIEW: Shuffles a deck of playing cards and deals a number of cards.

For each count argument, prints a line of tab-delimited cards to stdout,
or if there aren't enough cards remaining,
prints "Not enough cards" to stderr and exits with a nonzero status.

USAGE: deal [<count> ...]

ARGUMENTS:
  <count>                 The number of cards to deal at a time.

OPTIONS:
  -h, --help              Show help information.

Part "USAGE: deal" is misleading. It should be "USAGE: dealer".