msun: Correct FreeBSD version in sincos() man page

The sincos() man page notes the function was added to msun in FreeBSD
9.0 which must have been an oversight in the review as it was commited
to 12.0 and then backported to the 11 branch.

So I have provided a diff to correct this to the first FreeBSD version
it did ship with which was 11.2.

Reviewed by: dim, imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D40308