Update resolvers.md (#494)

## Write resolvers on types

the type in the schema should be Mission not Mutation


_src/schema.js_
```js
  type Mission{
    # ... with rest of schema
    missionPatch(mission: String, size: PatchSize): String
  }
```

docs/source/tutorial/resolvers.md

@@ -255,7 +255,7 @@ Mission: {
 
 _src/schema.js_
 ```js
-  type Mutation {
+  type Mission {
     # ... with rest of schema
     missionPatch(mission: String, size: PatchSize): String
   }