[Example] Add flow.flex.yaml for chat-minimal (#3572)

# Description

Fix `pf run create -f .\run.yml` error when loading flow.

# All Promptflow Contribution checklist:
- [ ] **The pull request does not introduce [breaking changes].**
- [ ] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
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license.**
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## General Guidelines and Best Practices
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- [ ] 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.

Signed-off-by: Brynn Yin <[email protected]>

examples/flex-flows/chat-minimal/flow.flex.yaml

@@ -0,0 +1,8 @@
+$schema: https://azuremlschemas.azureedge.net/promptflow/latest/Flow.schema.json
+entry: flow:chat
+sample: 
+  inputs:
+    question: What's Azure Machine Learning?
+environment:
+  # image: mcr.microsoft.com/azureml/promptflow/promptflow-python
+  python_requirements_txt: requirements.txt