Docs: Use modified_model in Composites example

[rodrigobdz]

In the with statement, the variable modified_model was not used to compute the output and the original model was used instead.

[chr5tphr]

Hey Rodrigo,

thank you for the fix!
Just as a side note, both model and modified_model describe the same object, since composites (and also attributors) temporarily change models in-place for the duration of the context. The context variable is used for clarity.

[rodrigobdz]

@chr5tphr Glad to help!
I see, thanks for the clarification.