Update internals.rst

The proper idiom is "behind the scenes".

I also fixed the sentence structure.
minor #4034 Update internals.rst (redstar504)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4034).

Discussion
----------

Update internals.rst

The proper idiom is "behind the scenes".

I also fixed the sentence structure.

Commits
-------

9966b90 Update internals.rst