Fix deployment bucket SSE documentation

[medikoo]

Regression introduced with #6547

If we set serverSideEncryption to true, then deployment will fail with:

 ServerlessError: Expected params.ServerSideEncryption to be a string
      at /Users/medikoo/npm-packages/serverless/lib/plugins/aws/provider/awsProvider.js:317:27
      at processTicksAndRejections (internal/process/task_queues.js:93:5)
      at runNextTicks (internal/process/task_queues.js:62:3)
      at processImmediate (internal/timers.js:412:9)
      at process.topLevelDomainCallback (domain.js:130:23)

It's expected to be encryption method.

It's also documented that way on AWS services page: https://github.com/serverless/serverless/blob/9a81af586f04ea9d4ea1fb38f44eec565d9b0365/docs/providers/aws/guide/services.md#serverlessyml

[codecov-io]

Codecov Report

Merging #7000 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7000   +/-   ##
=======================================
  Coverage   88.45%   88.45%           
=======================================
  Files         229      229           
  Lines        8417     8417           
=======================================
  Hits         7445     7445           
  Misses        972      972

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[astuyve]

Number 7000!!!