Adds a missing BIP 141 segwit BIP reference

Adding a missing reference to BIP 141 for clarity.

Author: iamsamwood

bip-0144.mediawiki

@@ -77,7 +77,7 @@ Parsers supporting this BIP will be able to distinguish between the old serializ
 
 If the witness is empty, the old serialization format should be used. 
 
-Witness objects can be extended to contain other kinds of witness data not yet specified. This BIP is agnostic to the witness data structure, defined in BIPxxxx (Consensus Segwit BIP), and treats it as an arbitrary byte array.
+Witness objects can be extended to contain other kinds of witness data not yet specified. This BIP is agnostic to the witness data structure, defined in [https://github.com/bitcoin/bips/blob/master/bip-0141.mediawiki BIP141] (Consensus segwit BIP), and treats it as an arbitrary byte array.
 
 * '''Rationale for not having an independent message type with its own serialization''': this would require separate "tx" and "block" messages, and all RPC calls operating on raw transactions would need to be duplicated, or need inefficinent or nondeterministic guesswork to know which type is to be used.