changing <pre> tags to markdown

Tone/source/oscillator/AMOscillator.ts

@@ -16,8 +16,7 @@ export interface AMOscillatorOptions extends ToneOscillatorOptions {
  * An amplitude modulated oscillator node. It is implemented with
  * two oscillators, one which modulators the other's amplitude
  * through a gain node.
- *
- * <pre>
+ * ```
  *    +-------------+       +----------+
  *    | Carrier Osc +>------> GainNode |
  *    +-------------+       |          +--->Output
@@ -25,7 +24,7 @@ export interface AMOscillatorOptions extends ToneOscillatorOptions {
  * +---------------+    |   +----------+
  * | Modulator Osc +>---+
  * +---------------+
- * </pre>
+ * ```
  *
  * @param frequency The starting frequency of the oscillator.
  * @param type The type of the carrier oscillator.

Tone/source/oscillator/FMOscillator.ts

@@ -14,7 +14,7 @@ export interface FMOscillatorOptions extends ToneOscillatorOptions {
 
 /**
  * FMOscillator implements a frequency modulation synthesis
- * <pre>
+ * ```
  *                                              +-------------+
  * +---------------+        +-------------+     | Carrier Osc |
  * | Modulator Osc +>-------> GainNode    |     |             +--->Output
@@ -24,7 +24,8 @@ export interface FMOscillatorOptions extends ToneOscillatorOptions {
  * +-----------------+   |
  * | modulationIndex +>--+
  * +-----------------+
- * </pre>
+ * ```
+ *
  * @param frequency The starting frequency of the oscillator.
  * @param type The type of the carrier oscillator.
  * @param modulationType The type of the modulator oscillator.

Tone/source/oscillator/PulseOscillator.ts

@@ -17,7 +17,7 @@ export interface PulseOscillatorOptions extends ToneOscillatorOptions {
  * also known as the duty cycle. At 50% duty cycle (width = 0) the wave is
  * a square wave.
  * [Read more](https://wigglewave.wordpress.com/2014/08/16/pulse-waveforms-and-harmonics/).
- * <pre>
+ * ```
  *    width = -0.25        width = 0.0          width = 0.25
  *
  *   +-----+            +-------+       +    +-------+     +-+
@@ -40,8 +40,7 @@ export interface PulseOscillatorOptions extends ToneOscillatorOptions {
  *       | |                                         | |
  *       | |                                         | |
  * +-----+ +-------+                                 +-+
- *
- * </pre>
+ * ```
  * @param frequency The frequency of the oscillator
  * @param width The width of the pulse
  * @example