Update security.rst

I'm not a native speaker but I think that it sounds better by adding that before the word `start`.
Improved the sentence
minor #3636 Update security.rst (nomack84)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3636).

Discussion
----------

Update security.rst

I'm not a native speaker but I think that it reads better now.

Commits
-------

1c2abab Improved the sentence
c8c093a Update security.rst