Update README.md

code style for err, numReplaced, upsert

README.md

@@ -268,7 +268,7 @@ db.count({}, function (err, count) {
 * `options` is an object with two possible parameters
   * `multi` (defaults to `false`) which allows the modification of several documents if set to true
   * `upsert` (defaults to `false`) if you want to insert a new document corresponding to the `update` rules if your `query` doesn't match anything
-* `callback` (optional) signature: err, numReplaced, upsert
+* `callback` (optional) signature: `err`, `numReplaced`, `upsert`
   * `numReplaced` is the number of documents replaced
   * `upsert` is set to true if the upsert mode was chosen and a document was inserted