Add missing comma. (graphql#412)

Author: davidgomes

site/learn/Learn-Execution.md

@@ -87,7 +87,7 @@ human(obj, args, context) {
 }
 ```
 
-The `context` is used to provide access to a database which is used to load the data for a user by the `id` provided as an argument in the GraphQL query. Since loading from a database is an asynchronous operation, this returns a [Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise). In JavaScript Promises are used to work with asynchronous values, but the same concept exists in many languages, often called *Futures*, *Tasks* or *Deferred*. When the database returns, we can construct and return a new `Human` object.
+The `context` is used to provide access to a database which is used to load the data for a user by the `id` provided as an argument in the GraphQL query. Since loading from a database is an asynchronous operation, this returns a [Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise). In JavaScript, Promises are used to work with asynchronous values, but the same concept exists in many languages, often called *Futures*, *Tasks* or *Deferred*. When the database returns, we can construct and return a new `Human` object.
 
 Notice that while the resolver function needs to be aware of Promises, the GraphQL query does not. It simply expects the `human` field to return something which it can then ask the `name` of. During execution, GraphQL will wait for Promises, Futures, and Tasks to complete before continuing and will do so with optimal concurrency.