Suggest that preview username should match LDAP
Suggest that preview username should match LDAP

Using the phrase "Let's say you want the username …" suggests it's a
free choice, but when people choose something that isn't their LDAP
username it's picked up in the pull request as something to fix.

Update the guidance to reflect that.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet