coveralls badge `branch` in URL query param

[owayss]

The coveralls badge has the branch URL query parameters set to the fixed string branch, instead of the user provided value for the branch attribute.

This is fixed in #68.

Current behavior:

For a badge configured like so:

$ tail -n1 Cargo.toml
coveralls = { repository = "owayss/huffman-encoding",  branch = "main" }

It configures the badge as:

$ cargo readme |grep coveralls
[![Coverage Status](https://coveralls.io/repos/github/owayss/huffman-encoding/badge.svg?branch=branch)](https://coveralls.io/github/owayss/huffman-encoding?branch=main)

Expected behavior

The branch URL query parameter should have the value provided for the branch attribute, that is, branch=main instead of branch=branch:

[![Coverage Status](https://coveralls.io/repos/github/owayss/huffman-encoding/badge.svg?branch=main)](https://coveralls.io/github/owayss/huffman-encoding?branch=main)