docs: onMutate should be async (TanStack#1585)

Since we await in the function body, onMutate should be an async function.

Author: hardfire

docs/src/pages/guides/mutations.md

@@ -216,7 +216,7 @@ const queryClient = new QueryClient()
 // Define the "addTodo" mutation
 queryClient.setMutationDefaults('addTodo', {
   mutationFn: addTodo,
-  onMutate: variables => {
+  onMutate: async (variables) => {
     // Cancel current queries for the todos list
     await queryClient.cancelQueries('todos')