fix: typo in example

[fernandezcuesta]

Description of your changes

There's a syntax error in the upjet external name documentation.

I have:

• [x] Read and followed Upjet's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [x] Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

n/a