mongodump --oplog not valid for config dump

1. Under "Backup one config server.", the mongodump step mistakenly uses -oplog with the -db config option.

mongodump --oplog --db config

Will opt to use just the --oplog option and not specify the database.

Affects: v2.4 + v2.6 + v3.0

2. Update the tutorial to reflect changes to mongodump for 3.0 – remove use of deprecated options and update steps accordingly.

Affects: v3.0