[Utility] Rename COMMANDS to POSITIONAL ARGUMENTS

- <rdar://problem/33851991> `swift run --help` says COMMANDS where it should say OPTIONS

Author: aciidgh

Sources/Utility/ArgumentParser.swift

@@ -895,7 +895,7 @@ public final class ArgumentParser {
 
  if positionalArguments.count > 0 {
  stream <<< "\n\n"
- stream <<< "COMMANDS:"
+ stream <<< "POSITIONAL ARGUMENTS:"
  for argument in positionalArguments.lazy.sorted(by: {$0.name < $1.name}) {
  guard let usage = argument.usage else { continue }
  print(formatted: argument.name, usage: usage, on: stream)