Fix incorrect schema example in Mutation page, add notes regarding nested input types (#3806)

* Update mutations.md

* Update input-object-types.md

* Update mutations.md

Author: seclerp

website/src/docs/hotchocolate/defining-a-schema/input-object-types.md

@@ -154,3 +154,5 @@ public class Startup
 
 </ExampleTabs.Schema>
 </ExampleTabs>
+
+⚠ Object types nested inside of an input object type need to also be declared as input types.

website/src/docs/hotchocolate/defining-a-schema/mutations.md

@@ -119,7 +119,7 @@ public class Startup
                   addBook(input: BookInput): Book
                 }
 
-                type BookInput {
+                input BookInput {
                   title: String
                   author: String
                 }