Fix #1897: WaveShaper curve setter allows multiple sets

Just delete the sentence that implise the curve can be set more than
once.  The algorithm is clear that you can only set it once.

index.bs

@@ -9440,8 +9440,7 @@ Attributes</h4>
 		When this attribute is set, an internal copy of the curve is
 		created by the {{WaveShaperNode}}. Subsequent
 		modifications of the contents of the array used to set the
-		attribute therefore have no effect: the attribute MUST be set
-		again in order to change the curve.
+		attribute therefore have no effect.
 
 		<div algorithm="WaveShaperNode.curve">
 			To set the {{WaveShaperNode/curve}} attribute, execute these steps: