pytest: fix a test that broke because of docstring usage

tests/test_plugin.py

@@ -196,7 +196,7 @@ def test_rpc_passthrough(node_factory):
     assert(len(cmd) == 1)
 
     # Make sure usage message is present.
-    assert only_one(n.rpc.help('hello')['help'])['command'] == 'hello [name]'
+    assert only_one(n.rpc.help('hello')['help'])['command'].startswith('hello [name]')
     # While we're at it, let's check that helloworld.py is logging
     # correctly via the notifications plugin->lightningd
     assert n.daemon.is_in_log('Plugin helloworld.py initialized')