`packer init' is meant to be invoked on a template file or a directory.
This was described in the command-line help message as
`[template.pkr.hcl|folder]'.
Because this is encapsulated between square brackets, this could be
taken as an option instead of a positional, mandatory argument.
This commit changes the wording to adopt a similar convention as `packer
build', so the two are consistent in how they are invoked, and to avoid
mistaking this argument as optional.
Lucas Bajolet
3c5ded984e