Fixing javadoc ascii table for ChannelFuture

Motivation:
The current ascii table [1] showing the various states that a ChannelFuture
can be in, but the table is slightly "off" for the 'cause'.

Modifications:
- Updated the ascii table to display nicely.

Result:
Easier to read the states of a ChannelFuture.

[1] http://netty.io/5.0/api/io/netty/channel/ChannelFuture.html