Fix comment in vendir-spec.md regarding path

[coreyti]

Before this change, the comment stated:

path is relative to vendir.yml location

That's not actually true. The path is actually relative to where vendir is executed. For example:

• given the following directory layout

  .
  ├── etc
  │   └── vendir.yml
  ├── opt
  └── src
    └── sub-directory

• and the following content in etc/vendir.yml

  ---
  apiVersion: vendir.k14s.io/v1alpha1
  kind: Config
  
  directories:
  - # path is relative to vendir.yml location (NOT TRUE)
  path: ../opt
  
  contents:
  - path: github.com/vmware-tanzu/carvel
  
    git:
      url: https://github.com/vmware-tanzu/carvel
      ref: origin/develop

• and the following shell execution

  cd ./src/sub-directory
  vendir sync --file ../../etc/vendir.yml

• the Carvel package will end up at ./src/opt/carvel, when the expectation (based on the comment) would be that it end up in ./opt/carvel.

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[pivotaljohn]

Excellent catch, @coreyti.

Thank you for taking the time to investigate and fix the docs! Few things are more frustrating than when the documentation is flat out wrong. 🙌🏻