[ADF] Fix syntax for {background: true} in $merge documentation

In the docs page for $merge, there is a section called "Run $merge in the background": https://www.mongodb.com/docs/atlas/data-federation/supported-unsupported/pipeline/merge/#run--merge-in-the-background

This page has the following syntax:

{
"$merge": {
"atlas":

{ "clusterName": "myTestCluster", "db": "sampleDB", "coll": "mySampleData" }

},

{ "background" : true }

}

However, it should say something like this:

db.<collection>.aggregate(
[

{ $merge: ... }

],

{ "background" : true }

)