Update docs/async.rst

putting "lambda chunk: body.put(chunk)" makes it feel like there is something misterious going on (are we putting "chunk" is some closure or something?).

And I found it difficult to understand without starting or running the worker.
remove confusing default arg, since request to localhost:8080/hello/ return 404
Merge pull request #559 from hvnsweeting/master

remove confusing default arg in index page
Merge pull request #465 from enriquepablo/master

Little documentation improvement (?)
Code cleanup. Removed deprecated code.
Less serious error message on abort(xxx) calls without second reason parameter.
Rendering none values as empty strings
Start of 0.13 development. Codename 'Friday'
Fixed changelog
Fixed changelog and made docs/conf.py py3k compatible.
Code cleanup. Removed APIs deprecated in 0.12
Fewer unclosed socket warnings in tests.
Merge pull request #563 from ctengiz/master

Rendering none values as empty strings for #562