io: show ErrUnexpectedEOF in ExampleReadAtLeast
io: show ErrUnexpectedEOF in ExampleReadAtLeast

Fixes #36245

Change-Id: I10ce50b0cc28b15f4e7be85b8f12cf9d0e4fac96
Reviewed-on: https://go-review.googlesource.com/c/go/+/212404
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>

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