Make use of build vs. builder consistent in mutations page

Author: mungojam

docs/rtk-query/usage/mutations.mdx

@@ -16,7 +16,7 @@ Mutations are used to send data updates to the server and apply the changes to t
 
 ## Defining Mutation Endpoints
 
-Mutation endpoints are defined by returning an object inside the `endpoints` section of `createApi`, and defining the fields using the `builder.mutation()` method.
+Mutation endpoints are defined by returning an object inside the `endpoints` section of `createApi`, and defining the fields using the `build.mutation()` method.
 
 Mutation endpoints should define either a `query` callback that constructs the URL (including any URL query params), or [a `queryFn` callback](./customizing-queries.mdx#customizing-queries-with-queryfn) that may do arbitrary async logic and return a result. The `query` callback may also return an object containing the URL, the HTTP method to use and a request body.