[minor] pkl crashes when using a -o when you mean a -m for multiple output { files }

[mobileskyfi]

Reporting as instructed... Happens with a simple .pkl file:

name = "test"
output {
    files {
        ["\(name).txt"] {
            text = "test"
        }
      } 
}

I came here to report & saw 0.28 release. Tried it, but same crash. Here is the stack trace from with --version of Pkl 0.28.0 (macOS 14.3.1, native):

pkl eval ./Specs/test.pkl -o './Machines'
An unexpected error has occurred. Would you mind filing a bug report?

java.nio.file.FileSystemException: /Users/amm0/PklPlist/Machines: Is a directory
        at java.base@21.0.5/sun.nio.fs.UnixFileSystemProvider.newByteChannel(UnixFileSystemProvider.java:261)
        at java.base@21.0.5/java.nio.file.spi.FileSystemProvider.newOutputStream(FileSystemProvider.java:482)
        at java.base@21.0.5/java.nio.file.Files.newOutputStream(Files.java:227)
        at java.base@21.0.5/java.nio.file.Files.write(Files.java:3492)
        at java.base@21.0.5/java.nio.file.Files.writeString(Files.java:3714)
        at org.pkl.commons.PathsKt.writeString(Paths.kt:51)
        at org.pkl.commons.PathsKt.writeString$default(Paths.kt:46)
        at org.pkl.cli.CliEvaluator.writeOutput(CliEvaluator.kt:158)
        at org.pkl.cli.CliEvaluator.doRun(CliEvaluator.kt:100)
        at org.pkl.commons.cli.CliCommand.run(CliCommand.kt:45)
        at org.pkl.cli.commands.EvalCommand.run(EvalCommand.kt:84)
        at com.github.ajalt.clikt.core.CoreCliktCommandKt.parse(CoreCliktCommand.kt:107)
        at com.github.ajalt.clikt.core.CoreCliktCommandKt.main(CoreCliktCommand.kt:78)
        at com.github.ajalt.clikt.core.CoreCliktCommandKt.main(CoreCliktCommand.kt:90)
        at org.pkl.cli.Main.main$lambda$0(Main.kt:26)
        at org.pkl.commons.cli.CliMainKt.cliMain(CliMain.kt:33)
        at org.pkl.cli.Main.main(Main.kt:26)

While I did read this in docs:

It is sometimes desirable for a single module to produce multiple output files. This is possible by configuring a module’s output.files property and specifying the --multiple-file-output-path (or -m for short) CLI option.

I forgot that part... I suppose it should CLI could suggest you need a -m instead, especially if given a a directory.