Fix pseudocode on bip-0158

[AlexITC]

The golomb_encode function should encode r instead of x.

[luke-jr]

@Roasbeef @aakselrod

[AlexITC]

I'm not sure what the Proposed BIP modification tag is about, I'm not proposing to change the BIP but fixing a minor detail on the pseudocode.

[matthewleon]

This appears to be an unnecessary change to the pseudocode. See the Bitcoin core implementation: https://github.com/bitcoin/bitcoin/blob/667a8617418d837092de5b37568d60b372519462/src/blockfilter.cpp#L37

[AlexITC]

@mattBlackDesign in fact, it wasn't obvious to me that this line is writing the P least significant bits only, which is what I basically made clear: write_bits_big_endian(stream, r, P).

I'm closing the ticket.