math/big: typo in formula used in comment

[gaufung]

Please answer these questions before submitting your issue. Thanks!
At line 85 in src/math/big/arith.go file, it should be q = (u1<<_W + u0 - r)/v rather than q = (u1<<_W + u0 - r)/y

What version of Go are you using (go version)?

go version go1.10.1 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

What did you expect to see?

What did you see instead?

[gopherbot]

Change https://golang.org/cl/145280 mentions this issue: math/big: fix a formula used as documentation

[gaufung]

@ALTree Thanks a lot