cbstats --help text needs to be updated

cbstats --help needs to be updated.  We don't use default bucket any longer, and there are no such things as bucket passwords either; the default user shouldn't be a bucket name.

Options:
-h, --help show this help message and exit
-a iterate over all buckets
-b BUCKETNAME the bucket to get stats from (Default: default)
-u USERNAME the user as which to authenticate (Default: bucketName)
-p PASSWORD the password for the bucket if one exists
-S read password from stdin
-j output the results in json format
-8 Force use of UTF8 symbols in output