Correct alphaToCoverage type (mrdoob#24931)

Author: WestLangley

docs/api/en/materials/Material.html

@@ -36,7 +36,7 @@ <h3>[property:Float alphaTest]</h3>
 		Default is `0`.
 		</p>
 
-		<h3>[property:Float alphaToCoverage]</h3>
+		<h3>[property:Boolean alphaToCoverage]</h3>
 		<p>
 		Enables alpha to coverage. Can only be used with MSAA-enabled contexts (meaning when the renderer was created with `antialias` parameter set to `true`).
 		Default is `false`.

docs/api/it/materials/Material.html

@@ -36,7 +36,7 @@ <h3>[property:Float alphaTest]</h3>
 			Il valore predefinito è `0`.
 		</p>
 
-		<h3>[property:Float alphaToCoverage]</h3>
+		<h3>[property:Boolean alphaToCoverage]</h3>
 		<p>
 			Abilita l'alfa alla copertura. Può essere utilizzato solo con contesti abilitati per MSAA (ovvero quando il renderer è stato creato con 
 			il parametro `antialias` impostato a `true`).

docs/api/zh/materials/Material.html

@@ -28,7 +28,7 @@ <h3>[property:Float alphaTest]</h3>
 <p>设置运行alphaTest时要使用的alpha值。如果不透明度低于此值，则不会渲染材质。默认值为*0*。
 </p>
 
-<h3>[property:Float alphaToCoverage]</h3>
+<h3>[property:Boolean alphaToCoverage]</h3>
 <p>
 启用alpha to coverage. 只能在开启了MSAA的渲染环境中使用 (当渲染器创建的时候*antialias* 属性要*true*才能使用).
 默认为 *false*.