Update constraints.md

[diegoquintanav]

Adds a reference to {{ target.schema }}, useful when defining foreign_key constraints.

What are you changing in this pull request and why?

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[mirnawong1]

hey @diegoquintanav great PR and thanks for opening it up to add to the docs! I've made some tweaks. there are some upcoming changes to foreign key constraints that will also make this more efficient (see github issue): Foreign key constraints support to (with ref) + to_columns

looking forward to future contributions ! ✨