curl example missing line-continuation backslashes

This curl example is split across multiple lines but is missing line-continuation backslashes. As written, `--header` and `--json` will be treated as separate shell commands when copy-pasted.

_Originally posted by @copilot-pull-request-reviewer in https://github.com/influxdata/docs-v2/pull/7503#discussion_r3594999226_

Start with the curl example referenced in pull request #7503 and inspect how its multiline command is rendered. Add the missing line-continuation backslashes so the full example runs as one shell command, then verify the copied command treats --header and --json as arguments rather than separate commands.

Written by the indexing model from the issue text.