chore: fix typo in doc/discovery.md

[tzchenxixi]

What does this pull request do? Explain your changes. (required)

fix typo in doc/discovery.md

Specific updates (required)

How did you test each of these updates (required)

Does this pull request close any open issues?

Checklist:

• [x] Read the contribution guide
• [x] make runs successfully
• [x] All tests in ./test.sh pass
• [ ] README and other documentation updated
• [ ] Pending changelog updated

[tzchenxixi]

@leszko Hi, Could you please review this PR at your convenience? Thank you very much.