docs: update source directive code example

[JonasKruckenberg]

• Quick Checklist

• I have read the contributing guidelines
• I have written new tests, as applicable (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• I have added a changeset, if applicable

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• What is the new behavior (if this is a feature change)?

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this
  PR?)

• Other information:

[changeset-bot]

⚠️ No Changeset found

Latest commit: c33c37a

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[codecov]

Codecov Report

Merging #419 (c33c37a) into main (6d39377) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #419   +/-   ##
=======================================
  Coverage   96.38%   96.38%           
=======================================
  Files          31       31           
  Lines        1080     1080           
  Branches      222      222           
=======================================
  Hits         1041     1041           
  Misses         39       39           

Flag	Coverage Δ
imagetools-core	98.51% <ø> (ø)
rollup-plugin-imagetools	98.47% <ø> (ø)
vite-imagetools	82.51% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.