docs: fix simple typo, transation -> translation

[timgates42]

There is a small typo in doc/zone-class.rst.

Should read translation rather than transation.

Semi-automated pull request generated by
https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md

[codecov-io]

Codecov Report

Merging #616 (c8fdddd) into master (692e915) will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #616      +/-   ##
==========================================
+ Coverage   97.81%   97.89%   +0.07%     
==========================================
  Files         121      121              
  Lines       10081    10081              
==========================================
+ Hits         9861     9869       +8     
+ Misses        220      212       -8     

Impacted Files	Coverage Δ
dns/set.py	98.44% <0.00%> (+2.32%)	⬆️
dns/immutable.py	100.00% <0.00%> (+4.54%)	⬆️
dns/_immutable_attr.py	97.56% <0.00%> (+7.31%)	⬆️

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