fix(doubles): the misuse advice must name a helper that exists

[Chemaclass]

🤔 Background

Related #1229

Passing a quoted command line where a command name belongs reported is not a usable command name for mock; pass arguments after it, as in 'mock ls -l'. A bare mock is command not found — the rule this API's docs stress hardest — so following the advice got the reader nowhere.

The example was wrong for each caller as well: bashunit::spy takes a single name, bashunit::mock ls -l means "mock ls with -l as the replacement body", and bashunit::mock_sequence expects answers rather than arguments.

💡 Changes

• Name the namespaced helper at all three call sites, and advise naming the command alone — which is true for all of them, since mock already forwards the call's own arguments to the replacement
• Run the recommended form in the tests instead of only string-matching the message; string-matching is what let the invented mock survive
• Those new assertions check behaviour rather than captured output — under --simple the rendered failure is a one-character marker, which the first attempt tripped over