Make flags documentation explicit that field has to be named flags

[stevensonmt]

• My programming background is ... self taught novice
• I want to learn Elm because ... intro to functional paradigm, TEA makes web app design sensible to me
• I was confused by ... flags.

Specifically my confusion was the need for flags to be passed as an object with a field named flags. The current guide implies that flags is the concept term, but is not explicit that the runtime requires the actual name flags.