update docs about cache paths

[taylorsilva]

I tested this with

jobs:
  - name: cache
    plan:
      - task: cache
        config:
          platform: linux
          image_resource: &img
            type: mock
            source:
              mirror_self: true
          caches:
            - path: /tmp/cache/something
          run:
            path: sh
            args:
              - -cx
              - |
                ls -lah /tmp/
                ls -lah /tmp/cache

Should have made this change when I made #547. I just skimmed the code in concourse/concourse@2ba0164 and didn't notice the change applied to caches as well.