Update defining-models.md be consistent with capitalization of Attr types

SHould these be titlecased or lower case?
`string`, `number`, `boolean`, and `date`.

Here they are Capitalized but in 
CREATING CUSTOM TRANSFORMATIONS
section they are lower case.
Merge pull request #676 from eccegordo/patch-2

Update defining-models.md be consistent with capitalization of Attr type...