fixup! bulletproofs_generators_serialize doc

"plus one" seems to be wrong

Author: jonasnick

include/secp256k1_bulletproofs.h

@@ -46,10 +46,8 @@ SECP256K1_API secp256k1_bulletproofs_generators* secp256k1_bulletproofs_generato
  *  Args:        ctx: pointer to a context object
  *               gen: pointer to the generator set to be serialized
  *  Out:        data: pointer to buffer into which the generators will be serialized
- *  In/Out: data_len: the length of the `data` buffer. Should be initially set to at
- *                    least 33 times the number of generators plus one(33 * (num_gens + 1));
- *                    will be ser to 33 times the number of generators plus one
- *                    on successful return
+ *  In/Out: data_len: the length of the `data` buffer. Should be at least
+ *                    k = 33 * num_gens. Will be set to k on successful return
  *
  * TODO: For ease of review, this setting G = H0 is not included in this commit. We will
  * add it in a separate commit.