Fix typo in ircbot's default matrix-notifier.

* Add a test fixture for the ircbot plugin that shows the bug.
* Fix the bug.

Update: Extract original example from the documentation as
ircbot001.yaml and include in the doc via literalinclude.  Add a
second test case, ircbot002.yaml to demonstrate the bug fixed by this
patch.

Closes-Bug: #1290376
Change-Id: Ifcb96a6dab5ee923cee7d3e71bfe518f1e3d0ebc
Merge "Fix typo in ircbot's default matrix-notifier."