math: minor error in Bessel function documentation

[IsaacDunlap]

Just spotted a typo in jn.go, in the documentation for the function Yn (https://golang.org/pkg/math/#Yn)).

Currently, we have a typo where Y1 is written instead of Yn:

Y1(n, x < 0) = NaN
Y1(n, NaN) = NaN

The documentation should say:
Yn(n, x < 0) = NaN
Yn(n, NaN) = NaN

Also, the current documentation for Yn doesn't include the special case Yn(0, 0) = -Inf, which should be included for completeness I'd imagine.

[gopherbot]

CL https://golang.org/cl/35970 mentions this issue.