crypto/ecdsa: correct code comment.

The code comment mixed up max and min. In this case, min is correct
because this entropy is only used to make the signature scheme
probabilistic. (I.e. if it were fixed then the scheme would still be
secure except that key.Sign(foo) would always give the same result for a
fixed key and foo.)

For this purpose, 256-bits is plenty.

Fixes #16819.

Change-Id: I309bb312b775cf0c4b7463c980ba4b19ad412c36
Reviewed-on: https://go-review.googlesource.com/30153


Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>