correct IN usage in docstring

[smtheard]

This is to fix an issue I found within the docs. __in is not a valid dynamoDB operator, it has to be __is_in in order to work both for dynamoDB and for dyanmORM.

[smtheard]

@borgstrom any idea why the CI seems to be running for infinite time? it's been a day haha

[codecov-io]

Codecov Report

Merging #101 (d79ea82) into master (418a0ed) will increase coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
+ Coverage   96.62%   96.81%   +0.19%     
==========================================
  Files          11       11              
  Lines        1007     1005       -2     
==========================================
  Hits          973      973              
+ Misses         34       32       -2     

Impacted Files	Coverage Δ
dynamorm/model.py	99.45% <ø> (ø)
dynamorm/table.py	96.19% <0.00%> (+0.38%)	⬆️

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