msun: Fix math error in comment explaining y reduction

x = k + y for some integer k and |y| < 1/2
exp2(x) = exp2(k + y) = exp2(k) * exp2(y)
which can be written as 2**k * exp2(y)

The original had x = 2k + y, which is has an extra 2 in it which
isn't correct.

Confirmed by forumula 2 in Gal and Bachelis referenced in the comments
for the source of this method
https://dl.acm.org/doi/pdf/10.1145/103147.103151

The actual code is correct.

Reviewed by: imp (who added s_exp2.c and wrote the commit message)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1127