fix typo in event_stream.py (microsoft#363)

# Description

splited -> splitted

# All Promptflow Contribution checklist:
- [x] **The pull request does not introduce [breaking changes]**
- [ ] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
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- [x] Title of the pull request is clear and informative.
- [x] There are a small number of commits, each of which have an
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### Testing Guidelines
- [ ] Pull request includes test coverage for the included changes.

docs/media/how-to-guides/how-to-enable-streaming-mode/scripts/event_stream.py

@@ -44,7 +44,7 @@ def append_line(self, line):
 
         parts = line.split(':', maxsplit=1)
         if len(parts) < 2:
-            raise ValueError("Bad format: Each line must could be splited into two parts by ':'.")
+            raise ValueError("Bad format: Each line must could be splitted into two parts by ':'.")
 
         prefix = parts[0]
         data = parts[1].strip()