Update README.md: replace "master" by "main"

s/master/main

[ ] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
[ ] Ensure that the pull request title represents the desired changelog entry
[ ] Please describe what you did
[ ] Link to relevant issues in GitHub or Jira
[ ] Link to relevant pull requests, esp. upstream and downstream changes
[ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

jenkinsci / opentelemetry-plugin