bridge_with_matrix: Fix test.
It was broken by commit
@@ -18,7 +18,7 @@ sample_config_path = "matrix_test.conf"
sample_config_text = """[matrix]
host = https://matrix.org
-username = @username@matrix.org
+username = @username:matrix.org
password = password
room_id = #zulip:matrix.org
