cephadm: add format-black reminder comment

Add a comment to tox.ini for people who might not know about the
relatively new `check-black` and `format-black` envs. While check-black
is part of the default envlist (as it is read-only) someone may not
know about the read-write version `format-black`.

Signed-off-by: John Mulligan <[email protected]>

Author: phlogistonjohn

src/cephadm/tox.ini

@@ -5,6 +5,8 @@ envlist =
     check-black
     py3
 skipsdist = true
+# REMINDER: run `tox -e format-black` to apply black formatting
+# with the exact same specs as `check-black` expects.
 
 [flake8]
 max-line-length = 100