Update custom_functions.md
Update custom_functions.md

Full names of weight and offset parameters of map() function to be more informative.
h2oai/h2o-dev,h2oai/h2o-dev,h2oai/h2o-3,h2oai/h2o-dev,michalkurka/h2o-3,michalkurka/h2o-3,michalkurka/h2o-3,h2oai/h2o-3,michalkurka/h2o-3,h2oai/h2o-dev,h2oai/h2o-dev,h2oai/h2o-3,michalkurka/h2o-3,h2oai/h2o-3,michalkurka/h2o-3,h2oai/h2o-3,h2oai/h2o-dev,h2oai/h2o-3,michalkurka/h2o-3,h2oai/h2o-3,h2oai/h2o-3,h2oai/h2o-dev