Update 12.3.md (#501)

Fixed a that if dstName file's length longer than srcName file io.Copy will cover only part content of srcName file.
If file srcName 's content is "abc", dstName's content is "1234" ,the result of io.Copy(dst, src) is "abc4" not "abc"

eBook/12.3.md

@@ -26,7 +26,7 @@ func CopyFile(dstName, srcName string) (written int64, err error) {
 	}
 	defer src.Close()
 
-	dst, err := os.OpenFile(dstName, os.O_WRONLY|os.O_CREATE, 0644)
+	dst, err := os.Create(dstName)
 	if err != nil {
 		return
 	}