fix doc: WebGLRendererParameters.alpha

by default it's false
https://github.com/mrdoob/three.js/blob/eb071dd079a328bfd156559ea1eeb619b6bda8e3/src/renderers/WebGLRenderer.js#L58

src/renderers/WebGLRenderer.d.ts

@@ -47,7 +47,7 @@ export interface WebGLRendererParameters {
 	precision?: string;
 
 	/**
-	 * default is true.
+	 * default is false.
 	 */
 	alpha?: boolean;