Update comment on how to enable debug log (#279)

To enable `core.debug` output, `ACTIONS_STEP_DEBUG` should be true instead of `ACTIONS_RUNNER_DEBUG`. https://docs.github.com/en/actions/managing-workflow-runs/enabling-debug-logging#enabling-step-debug-logging

src/main.ts

@@ -4,7 +4,7 @@ import {wait} from './wait'
 async function run(): Promise<void> {
   try {
     const ms: string = core.getInput('milliseconds')
-    core.debug(`Waiting ${ms} milliseconds ...`) // debug is only output if you set the secret `ACTIONS_RUNNER_DEBUG` to true
+    core.debug(`Waiting ${ms} milliseconds ...`) // debug is only output if you set the secret `ACTIONS_STEP_DEBUG` to true
 
     core.debug(new Date().toTimeString())
     await wait(parseInt(ms, 10))