Update function-callback.adoc (#1797)

turn onn => turn on

spring-ai-docs/src/main/antora/modules/ROOT/pages/api/function-callback.adoc

@@ -58,7 +58,7 @@ Use `java.util.Supplier<O>` or `java.util.function.Function<Void, O>` to define
 [source,java]
 ----
 FunctionCallback.builder()
-    .description("Turns light onn in the living room")
+    .description("Turns light on in the living room")
 	.function("turnsLight", () -> state.put("Light", "ON"))
     .inputType(Void.class)
 	.build();
@@ -237,4 +237,4 @@ FunctionCallback.builder()
 
 === Schema Mismatches
 * Ensure input types match the Function's input parameter types.
-* Use `ParameterizedTypeReference` for generic types.
+* Use `ParameterizedTypeReference` for generic types.