[docs]fix invalid option of pf flow validate (microsoft#3478)

# Description

`pf flow validate` don't have `file` option. The correct option is
`source` , right?

```bash
% pf flow validate -h
usage: pf flow validate [-h] --source SOURCE [--verbose] [-d]

Validate a flow and generate flow.tools.json for the flow.

options:
  -h, --help       show this help message and exit
  --source SOURCE  The flow or run source to be used.
  --verbose        Increase logging verbosity. Use --debug for full debug logs.
  -d, --debug      The flag to turn on debug mode for cli.

Examples:

# Validate flow
pf flow validate --source <path_to_flow>
```

# All Promptflow Contribution checklist:
- [x] **The pull request does not introduce [breaking changes].**
- [x] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
- [x] **I have read the [contribution guidelines](../CONTRIBUTING.md).**
- [x] **Create an issue and link to the pull request to get dedicated
review from promptflow team. Learn more: [suggested
workflow](../CONTRIBUTING.md#suggested-workflow).**

## General Guidelines and Best Practices
- [x] Title of the pull request is clear and informative.
- [x] There are a small number of commits, each of which have an
informative message. This means that previously merged commits do not
appear in the history of the PR. For more information on cleaning up the
commits in your PR, [see this
page](https://github.com/Azure/azure-powershell/blob/master/documentation/development-docs/cleaning-up-commits.md).

### Testing Guidelines
- [x] Pull request includes test coverage for the included changes.

Co-authored-by: Clement Wang <[email protected]>

docs/cloud/azureai/use-flow-in-azure-ml-pipeline.md

@@ -13,7 +13,7 @@ To enable this feature, customer need to:
     2. `run.yaml`: `$schema`: `https://azuremlschemas.azureedge.net/promptflow/latest/Run.schema.json`
 3. ensure that metadata has been generated and is up-to-date:
     1. `<my-flow-directory>/.promptflow/flow.tools.json` should exist;
-    2. customer may update the file via `pf flow validate --file <my-flow-directory>`.
+    2. customer may update the file via `pf flow validate --source <my-flow-directory>`.
 
 :::