dm: fix operate-source create description to match its actual behavior

[yahonda]

What is changed, added or deleted? (Required)

The usage example said "Use the following `operate-source` command to create a source configuration file", which describes the command backwards. The same document's own "Flags description" section (directly above) already states that `create` "Creates one or more upstream database sources" and that `config-file` "Specifies the file path of `source.yaml`" (an existing, pre-created input file) — i.e. the command registers an existing source file's contents into the DM cluster; it does not create the file.

Playground verification: deployed a live DM cluster (dm-master + dm-worker + dmctl v8.5.3) in a local Docker container and ran `operate-source create ./source.yaml`. No new file was written to disk, and `operate-source show` continued to report the source as registered even after `source.yaml` was deleted — confirming the command registers the source into cluster state rather than creating a file.

Source-code verification: `dm/ctl/master/operate_source.go` (`operateSourceFunc`) calls `common.GetFileContent(arg)` to read the existing `source.yaml`'s bytes, parses it with `config.SourceCfgFromYaml`, and sends the content over gRPC as `OperateSourceRequest{Config: contents, ...}`. The CLI never writes a file — it only reads one and transmits its content to dm-master.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

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Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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Reviewing files that changed from the base of the PR and between 8d85871 and 7db470c.

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• dm/dm-manage-source.md

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