fixed some small issues in the readme

[Mavv3006]

New Pull Request Checklist

• [x] I have read the Documentation
• [x] I have searched for a similar pull request in the project and found none
• [x] I have updated this branch with the latest develop to avoid conflicts (via merge from master or rebase)
• [x] I have added the required tests to prove the fix/feature I am adding
• [x] I have updated the documentation (if necessary)
• [x] I have run the tests and they pass

This merge request fixes / refers to the following issues: ... (no issue available)

Pull Request Description

This pull requests fixes some minor things I noticed in the readme. Included also the changed made in https://github.com/flutterchina/dio/pull/1548 for the english readme.