OptionParser documentation improvement (#6131)

Expanded the example to show how to handle invalid options.

src/option_parser.cr

@@ -19,6 +19,11 @@
 #   parser.on("-u", "--upcase", "Upcases the salute") { upcase = true }
 #   parser.on("-t NAME", "--to=NAME", "Specifies the name to salute") { |name| destination = name }
 #   parser.on("-h", "--help", "Show this help") { puts parser }
+#   parser.invalid_option do |flag|
+#     STDERR.puts "ERROR: #{flag} is not a valid option."
+#     STDERR.puts parser
+#     exit(1)
+#   end
 # end
 #
 # destination = destination.upcase if upcase