docs(types): correct Model.init return type

[sam-dt]

Pull Request check-list

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• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
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Description of change

Resolves #12112

Correct the return type of Model.init from void to Model to match to the return type defined in the API reference (which is the correct one).

[codecov]

Codecov Report

Merging #12148 into master will increase coverage by 21.48%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master   #12148       +/-   ##
===========================================
+ Coverage   74.85%   96.34%   +21.48%     
===========================================
  Files          86       95        +9     
  Lines        8373     9220      +847     
===========================================
+ Hits         6268     8883     +2615     
+ Misses       2105      337     -1768     

Impacted Files	Coverage Δ
lib/dialects/sqlite/index.js	100.00% <0.00%> (ø)
lib/dialects/sqlite/query.js	98.65% <0.00%> (ø)
lib/dialects/postgres/connection-manager.js	95.83% <0.00%> (ø)
lib/dialects/postgres/query.js	97.84% <0.00%> (ø)
lib/dialects/mariadb/connection-manager.js	100.00% <0.00%> (ø)
lib/dialects/sqlite/connection-manager.js	100.00% <0.00%> (ø)
lib/dialects/mariadb/query.js	87.78% <0.00%> (ø)
lib/dialects/postgres/index.js	100.00% <0.00%> (ø)
lib/dialects/mariadb/index.js	100.00% <0.00%> (ø)
lib/model.js	96.60% <0.00%> (+1.06%)	⬆️
... and 28 more

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