Macro `get_test_bin` documentation cut off

[MichaelMcDonnell]

I forgot to add a forward slash in the documentation of the new get_test_bin macro in 0.5.0. Steps to reproduce:

1. Open https://docs.rs/test_bin/latest/test_bin/macro.get_test_bin.html
2. Notice that the documentation says "tests" instead of "Returns the crate's binary as a Command that can be used for integration tests"