[BUILD] Fix python install instructions. (#3004)
[BUILD] Fix python install instructions. (#3004)

(cherry picked from commit 0af3c5ca18c91ef5bc424829dd16772302738340)
michalkurka/h2o-3,h2oai/h2o-3,h2oai/h2o-dev,h2oai/h2o-dev,h2oai/h2o-dev,michalkurka/h2o-3,h2oai/h2o-3,h2oai/h2o-dev,h2oai/h2o-3,michalkurka/h2o-3,h2oai/h2o-3,michalkurka/h2o-3,h2oai/h2o-3,h2oai/h2o-3,h2oai/h2o-dev,michalkurka/h2o-3,michalkurka/h2o-3,h2oai/h2o-dev,h2oai/h2o-dev,h2oai/h2o-3,michalkurka/h2o-3,h2oai/h2o-3