fix typo in README.md - Githubissues

What does this PR do?

This fixes a very minor typo in the readme: broswer => browser

[ ] added a test, or explain why one is not needed/possible...
[ ] no unrelated changes
[ ] asked someone for a code review
[ ] joined #karrot:matrix.org
[ ] added an entry to CHANGELOG.md (description, pull request link, username(s))
[ ] tried out on a mobile device (does everything work as expected on a smaller screen?)