docs: Update Minitest example to clarify it only works with Rails

[andyw8]

The example in the docs works with Rails but not plain minitest.

There are workarounds such as the m gem, or adding ActiveSupport to a non-Rails project, but I feel they are beyond the scope of the docs here.

Release Notes:

• N/A

[andyw8]

cc @vitallium

[maxdeviant]

Thanks!