The matrix integration folder in python-zulip-api changed its name
from matrix to bridge_with_matrix but this was not reflected in
the docs. Change the docs accordingly.
Solves issue #545 in the python-zulip-api directory.
Both the integrations use our new Matrix integration (with only one
additional paragraph for the IRC docs), so docs for both should point
to the same underlying Markdown macro for configuring a Matrix bridge.
This is a follow-up to #9491.