command: change wording for init's postitional arg

`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.

command/init.go

@@ -180,7 +180,7 @@ packer {
 
 func (*InitCommand) Help() string {
 	helpText := `
-Usage: packer init [options] [config.pkr.hcl|folder/]
+Usage: packer init [options] TEMPLATE
 
   Install all the missing plugins required in a Packer config. Note that Packer
   does not have a state.