headers: fix spelling error

src/handle_wrap.h

@@ -36,7 +36,7 @@ namespace node {
 // - Do not throw from handle methods. Set errno.
 //
 // - MakeCallback may only be made directly off the event loop.
-//   That is there can be no JavaScript stack frames underneith it.
+//   That is there can be no JavaScript stack frames underneath it.
 //   (Is there any way to assert that?)
 //
 // - No use of v8::WeakReferenceCallback. The close callback signifies that