SpringBoot Configuration sample in reference doc has wrong yaml formatting

This pr is just a modification of the code style of the yaml code block.

Comment From: snicoll

Thanks again @weixsun. I have reverted the quote change as discussed (not necessary and inconsistent with other configuration samples).

Comment From: philwebb

I think we should probably quote Strings in YAML. I tried to add a review note but I didn't click "submit" 🤦. I've opened #28709.