update service principals redirect (#1265)

## Description

<!-- ID for Jira ticket e.g [SPE-1234] -->

:ticket: [IPL-8702] https://hashicorp.atlassian.net/browse/IPL-8702

<!-- Add a brief description of changes here. Include any other
necessary relevant links -->
A redirect that was previously working no longer is. Notably there was a
redirect from the source URL to a destination that then was used as a
source for another redirect. For whatever reason chaining the redirects
in this way does not work (I don't know if this is supposed to be
supported or not), so this PR updates the first redirect so the
destination matches the second redirect.

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selecting the scope and urgency. -->

### Requested review scope:

- [x] Content touched by the PR _only_ (typos, clarifications, tips)
- [ ] Code test (command and code block changes)
- [ ] Flow and language near changes (new/rearranged steps)
- [ ] Review everything (rewrites, major changes)

### Review urgency:

- [x] ASAP (bug fixes, broken content, imminent releases)
- [ ] 3 days (small changes, easy reviews)
- [ ] 1 week (default)
- [ ] Best effort (very non-urgent)

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(or both if necessary) and delete the other one! -->

## All updates:

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I have:

- [ ] Verified that all status checks have passed
- [ ] Verified that preview environment has successfully deployed
- [ ] Verified appropriate `label` applied (`hcp` + `product name`)
- [ ] Added all required reviewers (code owners and external)

## Content checklist (optional)

Please do these things before requesting a review. I have:

- [ ] Made any associated code repositories public
- [ ] Added the `hashicorp-education/teamName` to any additional code or
example repos as repo admin
- [ ] Added redirects for any moved or removed pages
- [ ] Spell checked the tutorial(s)
- [ ] Followed the [unified style
guide](https://github.com/hashicorp/web-unified-docs/tree/main/docs/style-guide)
- [ ] Linted code snippets (Details per language
[here](https://github.com/hashicorp/engineering-docs/blob/master/writing/markdown.md#code-blocks))
- [ ] Checked the steps for completeness (no steps are implied or
hidden)
- [ ] Looked at the local or vercel build and checked each new or
changed page for:
  - display on the product curriculum page
  - callout box formatting
  - code block highlighting
  - right-hand navigation
  - next and back buttons
  - URL path


[SPE-1234]:
https://hashicorp.atlassian.net/browse/SPE-1234?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ
[IPL-8702]:
https://hashicorp.atlassian.net/browse/IPL-8702?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: jonathanfrappier

content/hcp-docs/redirects.jsonc

@@ -59,6 +59,11 @@
     "destination": "/hcp/docs/hcp/iam/service-principal",
     "permanent": true,
   },
+  {
+    "source": "/hcp/docs/hcp/iam/service-principals",
+    "destination": "/hcp/docs/hcp/iam/service-principal",
+    "permanent": true,
+  },
   {
     "source": "/hcp/docs/hcp/security/sso",
     "destination": "/hcp/docs/hcp/admin/iam/sso",