fix: README.md github app example secret

[efekete]

The configuration is expecting list of object, the given example has invalid JSON syntax.

Description of your changes

Fixes #

I have:

• [ ] Read and followed Crossplane's contribution process.
• [ ] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

[AtzeDeVries]

@efekete thanks.