[FAB-6093] Fix Kafka documentation

Fix formatting issues, a few typos, and remove redundant references
to Hyperledger association (see first paragraph in [1]).

Compare before [2] and after [3].

[1] https://lists.hyperledger.org/pipermail/hyperledger-tsc/2017-September/001086.html
[2] https://github.com/hyperledger/fabric/blob/master/docs/source/kafka.rst
[3] https://github.com/kchristidis/fabric/blob/fab-6093/docs/source/kafka.rst

Change-Id: Ife92e16a02b3e0739287bb7c304be849425c1a56
Signed-off-by: Kostas Christidis <kostas@christidis.io>