Correct intro re collection to strict:true

Fix error in basic introduction to Mongo DB, first example code given and commented, a feature of class Db method collection.

It isn't w:1 to make the driver check if the collection exists and issue an error if it does not.

The correct option is strict:true, as documented for the class.

Similarly for class Db method createCollection.

Observed to work with strict:true in 1.3.19, and not to work with w:1.