doc: update reference to removed method

[bendem]

Resolves #900

[adammike]

This looks appropriate to me, @briantist do you have any thoughts?

[briantist]

Also looks good to me!
@bendem could I trouble you to rebase? The CI should be fixed in #933

[bendem]

Sorry for the delay, was off the last few days. PR is rebased now. ;)

[codecov]

Codecov Report

Merging #942 (bab9495) into develop (a98cbf8) will increase coverage by 1.77%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #942      +/-   ##
===========================================
+ Coverage    81.64%   83.42%   +1.77%     
===========================================
  Files           65       65              
  Lines         3002     2968      -34     
===========================================
+ Hits          2451     2476      +25     
+ Misses         551      492      -59     

Impacted Files	Coverage Δ
hvac/v1/__init__.py	85.61% <ø> (-1.65%)	⬇️
hvac/api/vault_api_category.py	94.44% <0.00%> (-0.68%)	⬇️
hvac/adapters.py	86.45% <0.00%> (-0.28%)	⬇️
hvac/utils.py	77.06% <0.00%> (-0.21%)	⬇️
hvac/api/secrets_engines/identity.py	93.98% <0.00%> (-0.03%)	⬇️
hvac/api/auth_methods/mfa.py	100.00% <0.00%> (ø)
hvac/api/secrets_engines/kv.py	100.00% <0.00%> (ø)
hvac/api/system_backend/namespace.py	83.33% <0.00%> (+33.33%)	⬆️
hvac/api/secrets_engines/transform.py	72.65% <0.00%> (+42.96%)	⬆️