remove code sample that not exist as specifications in update method.

[ndjndj]

※ related to #720 that closed it due to my ineptitude.

Hello, thank you for maintenance of this gem.

Probably related to issue #657.

I found what incorrect code sample of update method in readme.md.

.update method does not accept conditions as arguments.

dynamoid/lib/dynamoid/persistence.rb

Line 232 in 6b96d1d

def update(hash_key, range_key_value = nil, attrs)
Therefore, the following code as shown in the sample will fail.

Address.update(id, { city: 'Chicago' }, if: { deliverable: true })
So I removed this code sample.

thank you!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6b96d1d) 90.89% compared to head (8a81cd2) 90.89%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #721   +/-   ##
=======================================
  Coverage   90.89%   90.89%           
=======================================
  Files          71       71           
  Lines        3414     3414           
=======================================
  Hits         3103     3103           
  Misses        311      311           

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[andrykonchin]

Thank you!