io: add error check to TeeReader Example
io: add error check to TeeReader Example

Change-Id: I0b94bdced47483c6412e9979ce2d103fbfc52afb
Reviewed-on: https://go-review.googlesource.com/c/go/+/353729
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Trust: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,golang/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,golang/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,golang/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go