curl example missing a line-continuation backslash

This curl example is split across multiple lines but is missing a line-continuation backslash after the `curl ...` line, and the backslash on the header line includes a trailing space. This breaks copy/paste in a shell.

_Originally posted by @copilot-pull-request-reviewer in https://github.com/influxdata/docs-v2/pull/7503#discussion_r3594999273_

Locate the curl example described in the issue, using pull request #7503 and its review discussion as context. Add the missing line-continuation backslash and remove the trailing space from the header-line backslash; done means the multi-line command can be copied and run correctly in a shell.

Written by the indexing model from the issue text.