Updated spec url

[brson]

The tendermint/spec repo is archived and directs people to the URL in this patch.

[codecov-commenter]

Codecov Report

Merging #1110 (64daf2a) into master (84012a7) will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1110     +/-   ##
========================================
- Coverage    64.7%   64.7%   -0.1%     
========================================
  Files         240     240             
  Lines       20822   20822             
========================================
- Hits        13490   13487      -3     
- Misses       7332    7335      +3     

Impacted Files	Coverage Δ
abci/src/codec.rs	87.8% <0.0%> (-1.4%)	⬇️
abci/src/server.rs	8.8% <0.0%> (-0.4%)	⬇️
light-client-verifier/src/types.rs	37.9% <0.0%> (-0.3%)	⬇️
testgen/src/header.rs	84.4% <0.0%> (+0.5%)	⬆️

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[thanethomson]

Thanks for catching this!