Change help text of LCB_INVALID_USERNAME to be more accurate

This would otherwise confuse users who are using a usename other than
'Adminsitrator'

Change-Id: I2129cf20eee15a789155035c4277ab3ec4ef8f7e
Reviewed-on: http://review.couchbase.org/43288
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>