Fix spelling issue in comment. (#26055)

Addresses #26039 (comment)

src/app/ReadHandler.cpp

@@ -179,7 +179,7 @@ void ReadHandler::OnInitialRequest(System::PacketBufferHandle && aPayload)
  StatusResponse::Send(status, mExchangeCtx.Get(), /* aExpectResponse = */ false);
  // At this point we can't have a persisted subscription, since that
  // happens only when ProcessSubscribeRequest returns success. And our
- // subscription id is almost certianly not actually useful at this
+ // subscription id is almost certainly not actually useful at this
  // point, either. So don't try to mess with persisted subscriptions in
  // Close().
  Close(CloseOptions::kKeepPersistedSubscription);