Bigtable: fix rendering of instance admin snippets.

[sangramql]

Closes #7522.

[sangramql]

@crwilcox @AVaksman @sduskis
Please review the code.

[tseaver]

@sangramql I can't see the failed Kokoro log, but @busunkim96 reports that it is:

Running session lint
Creating virtualenv using python3.7 in /tmpfs/src/github/google-cloud-python/bigtable/.nox/lint
pip install flake8 black ../api_core ../core
black --check google tests docs
 would reformat /tmpfs/src/github/google-cloud-python/bigtable/docs/snippets.py
All done! 💥 💔 💥
1 file would be reformatted, 72 files would be left unchanged.
Command black --check google tests docs failed with exit code 1 Session lint failed.

Can you please run nox -re blacken, commit, and push?

[sangramql]

nox -re blacken is working fine on my machine.
I also added 3.7 check.
@nox.session(python=["3.6","3.7"])
def blacken(session):
"""Run black.

--------------cmd--------------
MacBook-Pro:bigtable sqa$ nox -re blacken
nox > Running session blacken-3.6
nox > Re-using existing virtualenv at /Users/sqa/Documents/GitHub/google-cloud-python/bigtable/.nox/blacken-3-6.
nox > pip install --upgrade black
nox > black google tests docs
All done! ✨ 🍰 ✨
73 files left unchanged.
nox > Session blacken-3.6 was successful.
nox > Running session blacken-3.7
nox > Re-using existing virtualenv at /Users/sqa/Documents/GitHub/google-cloud-python/bigtable/.nox/blacken-3-7.
nox > pip install --upgrade black
nox > black google tests docs
All done! ✨ 🍰 ✨
73 files left unchanged.
nox > Session blacken-3.7 was successful.
nox > Ran multiple sessions:
nox > * blacken-3.6: success
nox > * blacken-3.7: success

[tseaver]

Ignoring spurious pubsub / logging / storage failures.