Update mutations.md (#3322)

Currently, this piece of code would fail when executing the following query:

```
mutation {
  fruit{
    add(input: {id: "this is only a example"}) {
      id
    }
  }
}
```

With the following error: 'NoneType' object has no attribute 'fruit', as it lacks the expected field type.

docs/general/mutations.md

@@ -169,7 +169,9 @@ class FruitMutations:
 
 @strawberry.type
 class Mutation:
-    fruit: FruitMutations
+    @strawberry.field
+    def fruit(self) -> FruitMutations:
+        return FruitMutations()
 ```
 
 <Note>