Fixed minor typo

instead of 'rake test' as shortcut, use 'rake'.

Closes #12780 [ci skip]

guides/source/testing.md

@@ -798,7 +798,7 @@ You don't need to set up and run your tests by hand on a test-by-test basis. Rai
 
 | Tasks                   | Description |
 | ----------------------- | ----------- |
-| `rake test`             | Runs all unit, functional and integration tests. You can also simply run `rake test` as Rails will run all the tests by default|
+| `rake test`             | Runs all unit, functional and integration tests. You can also simply run `rake` as Rails will run all the tests by default|
 | `rake test:controllers` | Runs all the controller tests from `test/controllers`|
 | `rake test:functionals` | Runs all the functional tests from `test/controllers`, `test/mailers`, and `test/functional`|
 | `rake test:helpers`     | Runs all the helper tests from `test/helpers`|