map: use correct load factor for deciding when to grow

map: use correct load factor for deciding when to grow

The correct load factor is 6.5, not 6.
This got broken by accident in CL 462115.

Fixes #63438

Change-Id: Ib07bb6ab6103aec87cb775bc06bd04362a64e489
Reviewed-on: https://go-review.googlesource.com/c/go/+/533279
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>