Update README.md to reflect proper syntax for docker container

Author: bschonec

Diff for: README.md

@@ -25,8 +25,8 @@ You can use this container by running `docker run --rm -v $(pwd):/repo puppet/pu
    - run `docker run --rm puppet/puppet-dev-tools:4.x onceover --help` to see builtin help
    - see [Onceover's readme](https://github.com/dylanratcliffe/onceover/blob/master/README.md) for details
 3. Rake tasks from the installed gems (see below)
-   - run a single rake task like so: `docker run --rm -v $(pwd):/repo puppet/puppet-dev-tools:4.x rake lint`
-   - run multiple rake tasks sequentially like so: `docker run --rm -v $(pwd):/repo puppet/puppet-dev-tools:4.x rake lint syntax yamllint`
+   - run a single rake task like so: `docker run --rm -v $(pwd):/repo puppet/puppet-dev-tools:4.x rake -f /Rakefile lint`
+   - run multiple rake tasks sequentially like so: `docker run --rm -v $(pwd):/repo puppet/puppet-dev-tools:4.x rake -f /Rakefile lint syntax yamllint`
 
 ### A note on Onceover usage