doc: fix typo in streams.md
Replaces the misplaced word 'in' with 'is' in description of Readable event:error
Checklist
- [x ] documentation is changed or added
- [ x] commit message follows commit guidelines
Affected core subsystem(s)
- doc
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Replaces the misplaced word 'in' with 'is' in description of Readable event:error