fix(ip6scale): Document correct prefix length

We are testing /128 in ip6 scale tests for a long time,
but we did not update the suite documentation when we switched.
This does that.
Note that ip6 bas (and feature) tests are still using /64.

Change-Id: I5e108497b3f516195fce0e480ca3ec18d204bd84
Signed-off-by: Vratko Polak <vrpolak@cisco.com>