Specify builder name using -only flag now possible

#### Overview of the Issue

Not really a bug but a documentation update request.

It looks like you are now able to use the builder name to specify which builder to use at run time using -only flag.

On the website, it says "Note: It is not yet possible to match a named build block to do this, but this is soon going to be possible. So here "a.*" will match nothing."

https://www.packer.io/docs/templates/hcl_templates/blocks/build

#### Reproduction Steps

I have a builder block with name as "main-builder". I am able to use the -only="main-builder" flag to specify the builder I want to use in the template.

### Packer version

Latest