Commit 604fa4d5 made TLS 1.0 the default minimum version. This commit
amends a comment to reflect that.

This is where the default is used in the absence of an explicit version
being set:
https://github.com/golang/go/blob/edadffa2f3464c48a234f3cf2fc092a03f91824f/src/crypto/tls/common.go#L391-L393

Change-Id: I8f1117ecdddc85bb1cc76a6834026505a380b793
Reviewed-on: https://go-review.googlesource.com/5525Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>