fixed acf3a944 from https://gitee.com/lanyill/docs/pulls/13646

修改globalAlpha属性demo
Signed-off-by: Nlanyill <lanyi3@huawei.com>

zh-cn/application-dev/reference/arkui-ts/ts-canvasrenderingcontext2d.md

@@ -388,7 +388,7 @@ struct GlobalAlpha {
         .height('100%')
         .backgroundColor('#ffff00')
         .onReady(() =>{
-        this.context.fillStyle = 'rgb(255,0,0)'
+          this.context.fillStyle = 'rgb(0,0,255)'
           this.context.fillRect(0, 0, 50, 50)
           this.context.globalAlpha = 0.4
           this.context.fillStyle = 'rgb(0,0,255)'

zh-cn/application-dev/reference/arkui-ts/ts-offscreencanvasrenderingcontext2d.md

@@ -407,7 +407,7 @@ struct GlobalAlpha {
         .height('100%')
         .backgroundColor('#ffff00')
         .onReady(() =>{
-          this.offContext.fillStyle = 'rgb(255,0,0)'
+          this.offContext.fillStyle = 'rgb(0,0,255)'
           this.offContext.fillRect(0, 0, 50, 50)
           this.offContext.globalAlpha = 0.4
           this.offContext.fillStyle = 'rgb(0,0,255)'