DOCS-11615: Correct example using collMod to add/update validation for an existing collection

Author: stennie

source/reference/command/collMod.txt

@@ -254,7 +254,7 @@ The following example adds a validator to an existing collection named
 
 .. code-block:: javascript
 
-   db.createCollection( "contacts", {
+   db.runCommand( { collMod: "contacts",
       validator: { $jsonSchema: {
          bsonType: "object",
          required: [ "phone" ],