Fix WebAudio#1855: Set default number of channels to 2 for SPN

Just say the number of channels defaults to 2 for both the input and
output.

index.bs

@@ -1054,8 +1054,8 @@ Methods</h4>
 
 		<pre class=argumentdef for="BaseAudioContext/createScriptProcessor(bufferSize, numberOfInputChannels, numberOfOutputChannels)">
 		bufferSize: The {{ScriptProcessorNode/bufferSize}} parameter determines the buffer size in units of sample-frames. If it's not passed in, or if the value is 0, then the implementation will choose the best buffer size for the given environment, which will be constant power of 2 throughout the lifetime of the node. Otherwise if the author explicitly specifies the bufferSize, it <em class="rfc2119" title="MUST">MUST</em> be one of the following values: 256, 512, 1024, 2048, 4096, 8192, 16384. This value controls how frequently the {{ScriptProcessorNode/onaudioprocess}} event is dispatched and how many sample-frames need to be processed each call. Lower values for {{ScriptProcessorNode/bufferSize}} will result in a lower (better) <a href="#latency">latency</a>. Higher values will be necessary to avoid audio breakup and <a href="#audio-glitching">glitches</a>. It is recommended for authors to not specify this buffer size and allow the implementation to pick a good buffer size to balance between <a href="#latency">latency</a> and audio quality. If the value of this parameter is not one of the allowed power-of-2 values listed above, <span class="synchronous">an {{IndexSizeError}} <em class="rfc2119" title="MUST">MUST</em> be thrown</span>.
-		numberOfInputChannels: This parameter determines the number of channels for this node's input. Values of up to 32 must be supported. <span class="synchronous">A {{NotSupportedError}} must be thrown if the number of channels is not supported.</span>
-		numberOfOutputChannels: This parameter determines the number of channels for this node's output. Values of up to 32 must be supported. <span class="synchronous">A {{NotSupportedError}} must be thrown if the number of channels is not supported.</span>
+		numberOfInputChannels: This parameter determines the number of channels for this node's input. The default value is 2. Values of up to 32 must be supported. <span class="synchronous">A {{NotSupportedError}} must be thrown if the number of channels is not supported.</span>
+		numberOfOutputChannels: This parameter determines the number of channels for this node's output. The default value is 2. Values of up to 32 must be supported. <span class="synchronous">A {{NotSupportedError}} must be thrown if the number of channels is not supported.</span>
 		</pre>
 
 		<div>