Fix math in BN_bn2dec comment.

The bound on log(2)/3 on the second line is incorrect and has an extra
zero compared to the divisions in the third line. log(2)/3 = 0.10034...
which is bounded by 0.101 and not 0.1001. The divisions actually
correspond to 0.101 which is fine. The third line also dropped a factor
of three.

The actual code appears to be fine. Just the comments are wrong.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>