docs: Fix invalid references to targets in discovery.relabel

[ajalab]

The component discovery.relabel exports a label output, but some code examples are trying to get targets field instead and failing.

This PR fixes such invalid references.

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[korniltsev]

LGTM thanks!

[github-actions]

The backport to release/v1.5 failed:

Validation Failed: "Could not resolve to a node with the global id of 'T_kwDOAG3Mbc4AczmP'."

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-3153-to-release/v1.5 origin/release/v1.5
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 65be0e07d548a7f6e048fb419f9a0c8f58bb9465

When the conflicts are resolved, stage and commit the changes:

git add . && git cherry-pick --continue

If you have the GitHub CLI installed:

# Push the branch to GitHub:
git push --set-upstream origin backport-3153-to-release/v1.5
# Create the PR body template
PR_BODY=$(gh pr view 3153 --json body --template 'Backport 65be0e07d548a7f6e048fb419f9a0c8f58bb9465 from #3153{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title "[release/v1.5] docs: Fix invalid references to `targets` in discovery.relabel" --body-file - --label "type/docs" --label "backport" --base release/v1.5 --milestone release/v1.5 --web

Or, if you don't have the GitHub CLI installed (we recommend you install it!):

# Push the branch to GitHub:
git push --set-upstream origin backport-3153-to-release/v1.5

# Create a pull request where the `base` branch is `release/v1.5` and the `compare`/`head` branch is `backport-3153-to-release/v1.5`.

# Remove the local backport branch
git switch main
git branch -D backport-3153-to-release/v1.5