Fix coveralls badge config

[owayss]

This PR:

• https://github.com/livioribeiro/cargo-readme/commit/d2db80b48890a143d9d5639c198c9ce3a31e6ba2 replaces String::split_off(usize) with String::truncate(usize) as its returned value is unused so that the tests no_entrypoint_fail and multiple_bin_fail have the expected output.

• https://github.com/livioribeiro/cargo-readme/commit/6fed7ed15d6c72c795881c3411f4c2492c0f5e86 fixes the coveralls badge config so it uses the configured branch in the branch attribute.

Thanks for this crate! 🥳