Fix BC since Webpack5: The devtool option is more strict

[webda2l]

Tiny fix of error:

Serverless: Bundling with Webpack...
 
  Validation Error ---------------------------------------
 
  ValidationError: Invalid configuration object. Webpack has been initialized using a configuration object that does not match the API schema.
   - configuration.devtool should match pattern "^(inline-|hidden-|eval-)?(nosources-)?(cheap-(module-)?)?source-map$".
     BREAKING CHANGE since webpack 5: The devtool option is more strict.
     Please strictly follow the order of the keywords in the pattern.

See aligent/serverless-aws-nodejs-service-template#52 too

[medikoo]

Thank you @webda2l !