math/rand: fix uniform distribution stddev in tests
math/rand: fix uniform distribution stddev in tests

The standard deviation of a uniform distribution is size / √12.
The size of the interval [0, 255] is 256, not 255.
While we're here, simplify the expression.

The tests previously passed only because the error margin was large enough.
Sample observed standard deviations while running tests:

73.7893634666819
73.9221651548294
73.8077961697150
73.9084236069471
73.8968446814785
73.8684209136244
73.9774618960282
73.9523483202549

255 / √12 == 73.6121593216772
256 / √12 == 73.9008344562721

Change-Id: I7bc6cdc11e5d098951f2f2133036f62489275979
Reviewed-on: https://go-review.googlesource.com/51310
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,josharian/go,CAFxX/go,polyverse-security/go,akutz/go,Cofyc/go,codestation/go,nawawi/go,CAFxX/go,josharian/go,Cofyc/go,polyverse-security/go,golang/go,Cofyc/go,akutz/go,nawawi/go,golang/go,codestation/go,nawawi/go,akutz/go,cloudflare/go,polyverse-security/go,CAFxX/go,polyverse-security/go,cloudflare/go,CAFxX/go,josharian/go,Cofyc/go,codestation/go,cloudflare/go,codestation/go,josharian/go,CAFxX/go,akutz/go,Cofyc/go,CAFxX/go,akutz/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,josharian/go,cloudflare/go,golang/go,polyverse-security/go,polyverse-security/go,Cofyc/go,codestation/go,codestation/go,Cofyc/go,josharian/go,josharian/go,codestation/go,akutz/go,golang/go,codestation/go,cloudflare/go,akutz/go,josharian/go,cloudflare/go,cloudflare/go,polyverse-security/go,golang/go,nawawi/go,Cofyc/go,nawawi/go,nawawi/go,polyverse-security/go,golang/go,CAFxX/go,cloudflare/go,akutz/go,christopher-henderson/Go