docs: Improve the quality of the samples that target the admin client

[danieljbruce]

Description

During quality checks, additional errors with the samples were found. We should adjust the samples so that they are all correct.

Impact

Makes the samples work properly when they are run.

Testing

I extracted all the samples out that aren't covered in the tests and ran them one by one to find these mistakes. You can find the extracted samples in https://github.com/googleapis/nodejs-bigtable/tree/samples-examples.