first-run: keep 'bootstrap token' phrase so e2e log-scraper still matches

The CI e2e workflow greps for 'bootstrap token' in server logs to capture
the one-shot token. The earlier reword dropped that phrase; restore it on
the headless-instructions line so .gitea/workflows/e2e.yml step 'Capture
bootstrap token from server logs' keeps matching.

cmd/server/main.go

@@ -156,7 +156,7 @@ func run() error {
 		}
 		fmt.Fprintln(os.Stderr, "")
 		fmt.Fprintln(os.Stderr, "  Headless? POST {token, username, password} to /api/bootstrap")
-		fmt.Fprintln(os.Stderr, "  with this one-shot token (valid until first user is created):")
+		fmt.Fprintln(os.Stderr, "  with this one-shot bootstrap token (valid until first user exists):")
 		fmt.Fprintln(os.Stderr, "    "+token)
 		fmt.Fprintln(os.Stderr, "================================================================")
 	}