Tutorials (Pulse Scheduler): fix header levels (#1061)

Fix issue where "Scheduler Methods" would show up as a top-level header in the Pulse Tutorials on https://qiskit.org/documentation/tutorials/pulse/.

tutorials/pulse/6_pulse_scheduler.ipynb

@@ -352,7 +352,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "# Scheduler methods\n",
+    "## Scheduler methods\n",
     "\n",
     "The scheduler can follow multiple routines. By default, it follows an _as late as possible_ (ALAP) rule. Another scheduling method is _as soon as possible_, (ASAP). For both methods, the output schedule is minimal: in the longest-duration operation path of the input circuit, the start time of each operation is the end time of the proceeding operation. The methods determine how to schedule operations outside that longest path.\n",
     "\n",