Minor: typo "Slop" -> "Slot" and missing [[nodiscard]] on Signal::subscribe() #1159

In include/behaviortree_cpp/utils/signal.h:

1. Doc comment typo: "Signal/Slop implementation" should be "Signal/Slot implementation" (line 11).

2. Signal::subscribe() returns a Subscriber (a shared_ptr that is the only strong
   reference keeping the subscription alive; subscribers_ only holds a weak_ptr).
   If the caller discards the return value, the shared_ptr refcount drops to zero
   immediately, the weak_ptr expires, and the subscription is silently dropped before
   it can ever fire. Marking it [[nodiscard]] would catch this footgun at compile time:

   [[nodiscard]] Subscriber subscribe(CallableFunction func)

Happy to send a PR for both if useful.