fix(canvas-context): web平台代码警告的问题

pages/component/canvas/canvas-context.uvue

@@ -708,7 +708,7 @@
         context.lineWidth = 10;
         ['butt', 'round', 'square'].forEach((item : string, index : number) => {
           context.beginPath()
-          context.lineCap = item
+          context.lineCap = item as 'butt' | 'round' | 'square'
           context.moveTo(20, 20 + 20 * index)
           context.lineTo(100, 20 + 20 * index)
           context.stroke()
@@ -720,7 +720,7 @@
         context.lineWidth = 10;
         ['bevel', 'round', 'miter'].forEach((item : string, index : number) => {
           context.beginPath()
-          context.lineJoin = item
+          context.lineJoin = item as 'bevel' | 'round' | 'miter'
           context.moveTo(20 + 80 * index, 20)
           context.lineTo(100 + 80 * index, 50)
           context.lineTo(20 + 80 * index, 100)