fix wrong method in docs

[sinisaos]

Fixing a small mistake around encryption key generation in docs. XChaCha20Provider.generate_key() raise AttributeError: type object 'XChaCha20Provider' has no attribute 'generate_key' (because generate_key is Fernet method). I think it should be https://github.com/piccolo-orm/piccolo_api/blob/bc7eefa9d63b018cf45853798acbfd0be0978754/piccolo_api/encryption/providers.py#L179

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.02%. Comparing base (1157c12) to head (4f08f1f).
Report is 29 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #428      +/-   ##
==========================================
+ Coverage   93.42%   94.02%   +0.60%     
==========================================
  Files           5        5              
  Lines         365      402      +37     
==========================================
+ Hits          341      378      +37     
  Misses         24       24              

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[dantownsend]

@sinisaos Good catch, thanks!