math/rand: mention half-open intervals explicitly
math/rand: mention half-open intervals explicitly

If someone sees "in [0,n)" it might look like a typo.
Saying "in the half-open interval [0,n)" will give people
something to search the web for (half-open interval).

Change-Id: I3c343f0a7171891e106e709ca77ab9db5daa5c84
Reviewed-on: https://go-review.googlesource.com/c/go/+/328210
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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