Remove dollar sign prefix on code example (#1268)

The dollar sign prefix is included on the `$ docker build .` example in
step 5. Other examples do not include the dollar sign prefix, and so
this example is inconsistent.

.github/CONTRIBUTING.md

@@ -16,7 +16,7 @@ Please take moment and read this fantastic post on how to easily work with Go re
 2. If you want to implement a new feature that doesn't have an issue open. Please open one and ask for feedback on the feature before spending a lot of time working on it. It's possible the feature has already been discussed, or it's out of scope, or some other reason that might later prevent a PR from being accepted. The [#buffalo](https://gobuffalo.io/docs/slack) channel on gophers.slack.com is a great place to seek this kind of guidance.
 3. Write your feature/fix and make sure to include tests. Tests are an **absolute** requirement for any pull request. Please make sure to use the same testing style and libraries as the rest of the tests.
 4. Make sure tests run when doing `go test ./...`. You may need to do `go get -t ./...` first to get the testing dependencies.
-5. (Optional) There is a much longer set of integration tests that can be run. These will be run by Travis-CI when you open a PR. If you want to run them locally you can by running `$ docker build .`.
+5. (Optional) There is a much longer set of integration tests that can be run. These will be run by Travis-CI when you open a PR. If you want to run them locally you can by running `docker build .`.
 
 ### Making your Pull Request