minor #4845 Update security.rst (meelijane)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #4845).

Discussion
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Update security.rst

"setup" is a noun. "To set up" is a verb. "To set up" is an action statement, and should not use the noun form.

Commits
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70d5aba Update security.rst

book/security.rst

@@ -5,7 +5,7 @@ Security
 ========
 
 Symfony's security system is incredibly powerful, but it can also be confusing
-to setup. In this chapter, you'll learn how to setup your application's security
+to set up. In this chapter, you'll learn how to set up your application's security
 step-by-step, from configuring your firewall and how you load users to denying
 access and fetching the User object. Depending on what you need, sometimes
 the initial setup can be tough. But once it's done, Symfony's security system