Fix variable name styling in README and CHANGELOG

Overview

The wording in the README and CHANGELOG files in this project when adding optional variables is unclear. We should change the wording from:

Add optional variable(`REPORT_STDOUT`) to ...

to

Add optional variable `REPORT_STDOUT` to ...

Context

The following discussion from !31 (closed) should be addressed:

• @ayufan started a discussion:

  I actually don't understand why we use "word(variable". It seems that it should be:

  • "word (variable": added space,
  • or simpler "word variable": without (), with space.