Removed note on concurrency group/matrix usage

[james2791]

The note below in the build_matrix page indicates that you are able to use matrix setup dimension values inside concurrency groups (i.e name): like the below

steps:
- label: "A Matrix Build"
  command: "echo {{matrix.os}} {{matrix.arch}} {{matrix.test}}"
  concurrency: 1
  concurrency_group: "group-{{matrix.arch}}"
  matrix:
    setup:
      arch:
        - "amd64"
      os:
        - "macos"
      test:
        - "alpha"

That interpolation for the CG's name at the moment isn't supported - and will cause confusion for those hoping the dynamic naming/support can work in current form (either hard coded names/plugins are the way around this)

[mbelton-buildkite]

One minor nitpick, then this looks all good :)