contrib: Update contributing checklist for config

contributing/checklist-adding-config-fields.md

@@ -29,10 +29,10 @@ There are four specific cases covered with increasing complexity:
    `agent/config/runtime.go`.
  - [ ] Add an appropriate parser/setter in `agent/config/builder.go` to
    translate.
- - [ ] Add the new field with a random value to both the JSON and HCL blobs in
-   `TestFullConfig` in `agent/config/runtime_test.go`, it should fail now, then
-   add the same random value to the expected struct in that test so it passes
-   again.
+ - [ ] Add the new field with a random value to both the JSON and HCL files in
+   `agent/config/testdata/full-config.*`, which should cause the test to fail.
+   Then update the expected value in `TestLoad_FullConfig` in
+   `agent/config/runtime_test.go` to make the test pass again.
  - [ ] Run `go test -run TestRuntimeConfig_Sanitize ./agent/config -update` to update
    the expected value for `TestRuntimeConfig_Sanitize`. Look at `git diff` to
    make sure the value changed as you expect.