spec.yaml should be singular to match working yaml (#1674)

Co-authored-by: Carrie Roberts <clr2of8@gmail.com>

atomic_red_team/spec.yaml

@@ -116,7 +116,7 @@ atomic_tests:
   # per test, but there are cases where you may have multiple - for example, separate executors for `sh`
   # and `bash` when working on linux OSes.
   # Names of cloud/container specific runtimes can also be used, such as `aws`, `az`, `gcloud` and `kubectl`.
-  executors:
+  executor:
   # the name of the executor describes the framework or application in which the test should be executed.
   #
   # Each of these executors will have options that the executor needs to run. Possible executors we've imagined
@@ -200,6 +200,6 @@ atomic_tests:
     
   # in this example we have no input arguments
   input_arguments:
-  executors:
+  executor:
   - name: bash
     command: echo "Hello world!"