replaces call to insertOne with call to updateOne with upsert (#385) (#503)

* replaces call to insertOne with call to updateOne with upsert

* spacing fixes

Author: jmd-mongo

source/tutorial/modify-chunk-size-in-sharded-cluster.txt

@@ -34,7 +34,12 @@ To modify the chunk size, use the following procedure:
 
    .. code-block:: javascript
 
-      db.settings.insertOne( { _id:"chunksize", value: <sizeInMB> } )
+      db.settings.updateOne(
+         { _id: "chunksize" },
+         { $set: { _id: "chunksize", value: <sizeInMB> } },  
+         { upsert: true }
+      )
+
 
 Modifying the chunk size has several limitations: