fix a semantic error

[moooofly]

No description provided.

[hashicorp-cla]

All committers have signed the CLA.

[moooofly]

It seems not a real error.

=== RUN   TestRTTCommand_Run_BadArgs
    TestRTTCommand_Run_BadArgs: log.go:181: 2021/02/24 01:49:03 [INFO] agent: Serf agent starting
    TestRTTCommand_Run_BadArgs: log.go:181: 2021/02/24 01:49:03 [DEBUG] memberlist: Got bind error: Failed to start TCP listener on "127.0.0.77" port 7946: listen tcp 127.0.0.77:7946: bind: address already in use
    TestRTTCommand_Run_BadArgs: util_test.go:42: err: Error creating Serf: Failed to create memberlist: Could not set up network transport: failed to obtain an address: Failed to start TCP listener on "127.0.0.77" port 7946: listen tcp 127.0.0.77:7946: bind: address already in use
--- FAIL: TestRTTCommand_Run_BadArgs (0.10s)

[dnephin]

Thank you for the PR! I'm not sure why that test keeps failing, but it does pass locally for me on this branch and does not appear to be related to this change.

I made one other small change to the language in this error message.