Clarify an error message

Author: nedbat

coverage/cmdline.py

@@ -614,7 +614,7 @@ def do_run(self, options, args):
                 # they will be None if they have not been specified.
                 if getattr(options, opt_name) is not None:
                     self.help_fn(
-                        "Options affecting multiprocessing must be specified "
+                        "Options affecting multiprocessing must only be specified "
                         "in a configuration file."
                     )
                     return ERR

tests/test_cmdline.py

@@ -470,7 +470,7 @@ def test_multiprocessing_needs_config_file(self):
         # config file.
         self.command_line("run --concurrency=multiprocessing --branch foo.py", ret=ERR)
         self.assertIn(
-            "Options affecting multiprocessing must be specified in a configuration file.",
+            "Options affecting multiprocessing must only be specified in a configuration file.",
             self.stderr()
         )