Update erlang.mk and update paths to start the release

Fixes a couple invalid instructions at the same time.
Fix typo
Roadmap for Cowboy 2.0
Forgot a todo in the ROADMAP
Update specs that were too wide
Return 422 instead of 400 when AcceptResource returns false

422 is undefined for HTTP and interpreted as 400.
Add note about httpbis to the roadmap
Tweak the REST principles chapter following feedback
Add missing bracket in a guide example