message: fix `XOnlyPubKey::IsFullyValid` comment reference
  The method name `CreatePayToContract` doesn't exist, very likely it was
  a (local) working title that was renamed to `CreateTapTweak` later.
  Also mention `CheckTapTweak`.
diff: @@ -230,8 +230,8 @@ public:
  XOnlyPubKey& operator=(const XOnlyPubKey&) = default;
  /** Determine if this pubkey is fully valid. This is true for approximately 50% of all
  - * possible 32-byte arrays. If false, VerifySchnorr and CreatePayToContract will always
  - * fail. */
  + * possible 32-byte arrays. If false, VerifySchnorr, CheckTapTweak and CreateTapTweak
  + * will always fail. */
  bool IsFullyValid() const;
  /** Test whether this is the 0 key (the result of default construction). This implies
  