fix reference to incorrect usage of collStats command

correct usage in http://docs.mongodb.org/manual/reference/collection-statistics/

example:

    > db.runCommand({collStats:"foo"})
    {
    	"ns" : "test.foo",
    	"count" : 8,
    	"size" : 292,
    	"avgObjSize" : 36.5,
    	"storageSize" : 4096,
    	"numExtents" : 1,
    	"nindexes" : 1,
    	"lastExtentSize" : 4096,
    	"paddingFactor" : 1,
    	"systemFlags" : 1,
    	"userFlags" : 0,
    	"totalIndexSize" : 8176,
    	"indexSizes" : {
    		"_id_" : 8176
    	},
    	"ok" : 1
    }
    > db.runCommand({collStats:"test.foo"})
    { "ok" : 0, "errmsg" : "ns not found" }
    >