docs: fix spec links (#4384)

- erik fixed many of the broken links, just fixed two outstanding ones.

- closes #4381

Signed-off-by: Marko Baricevic <marbar3778@yahoo.com>

Co-authored-by: Anton Kaliaev <anton.kalyaev@gmail.com>

UPGRADING.md

@@ -232,14 +232,14 @@ due to changes in how various data structures are hashed.
 Any implementations of Tendermint blockchain verification, including lite clients,
 will need to be updated. For specific details:
 
-- [Merkle tree](./docs/spec/blockchain/encoding.md#merkle-trees)
-- [ConsensusParams](./docs/spec/blockchain/state.md#consensusparams)
+- [Merkle tree](https://github.com/tendermint/spec/blob/master/spec/blockchain/encoding.md#merkle-trees)
+- [ConsensusParams](https://github.com/tendermint/spec/blob/master/spec/blockchain/state.md#consensusparams)
 
 There was also a small change to field ordering in the vote struct. Any
 implementations of an out-of-process validator (like a Key-Management Server)
 will need to be updated. For specific details:
 
-- [Vote](https://github.com/tendermint/tendermint/blob/master/docs/spec/consensus/signing.md#votes)
+- [Vote](https://github.com/tendermint/spec/blob/master/spec/consensus/signing.md#votes)
 
 Finally, the proposer selection algorithm continues to evolve. See the
 [work-in-progress