Update canvas.md

修复变量未定义问题
```
var text1 = ctx.measureText('Hello world');
text.width // 49.46

ctx.font = 'Bold 20px Arial';
text2.width // 107.78
```

Author: yangxueyou

docs/canvas.md

@@ -315,9 +315,10 @@ var canvas = document.getElementById('myCanvas');
 var ctx = canvas.getContext('2d');
 
 var text1 = ctx.measureText('Hello world');
-text.width // 49.46
+text1.width // 49.46
 
 ctx.font = 'Bold 20px Arial';
+var text2 = ctx.measureText('Hello world');
 text2.width // 107.78
 ```