misc/wasm: update deprecated substr usage

String.prototype.substr is deprecated and usage is no longer
recommended so using String.prototype.substring instead.

Change-Id: I9eb49a8c065890df73301e3a04af59f550bc3ae1
Reviewed-on: https://go-review.googlesource.com/c/go/+/406094


TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>