misc/wasm: update deprecated substr usage
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 <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Auto-Submit: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go