RFR: 8335349: jcmd VM.classloaders fold option should be optional

[Kevin Walls]

Trivial tweak to ```ClassLoaderHierarchyDCmd::ClassLoaderHierarchyDCmd(outputStream* output, bool heap)``` to make _fold an optional argument.  

It still _is_ optional, but without this change it's not listed as optional in help.

The man page is already correct, and shows all the options as optional.

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Commit messages:
 - 8335349: jcmd VM.classloaders fold option should be optional

Changes: https://git.openjdk.org/jdk/pull/19948/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19948&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335349
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19948.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19948/head:pull/19948

PR: https://git.openjdk.org/jdk/pull/19948