Fix SchemaModel dtype example

[jeffzi]

In the documentation of schema models, the second note about supported dtypes is not rendered properly.

I changed the directive from code-block to testcode because :skipif: is not supported by code-block.

[codecov-io]

Codecov Report

Merging #309 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #309   +/-   ##
=======================================
  Coverage   98.64%   98.64%           
=======================================
  Files          18       18           
  Lines        1702     1702           
=======================================
  Hits         1679     1679           
  Misses         23       23           

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