improve error message when url scheme is not provided

[sanan-fataliyev]

Hey. Thank you for this library. It was hard to find the root cause of the issue until I carefully compare my code line-by-line with the example on README.

Code:

Test(t,
    Post("httpbin.org/get"),
    Expect().Status().Equal(http.StatusOK),
)

Outputs: Error: unable to create a request: did you called Post(), Get(), ...?

I think the error message can be improved for this use case.
Also, you can fix the tiny gramma mistake in the original message (did you called -> did you call)