Small typos in documentation of mlr nest

[kusalananda]

The documentation for the nest operation says:

  --evar {string}       Shorthand for --explode --values ---across-records --nested-fs {string}
  --ivar {string}       Shorthand for --implode --values ---across-records --nested-fs {string}

Notice the triple dashes in front of across-records (these should be double dashes).

Found in output of mlr help nest from Miller version mlr version 6.8.0-dev for openbsd/amd64/go1.20.1.

[johnkerl]

Thank you @kusalananda !