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
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Update security.rst

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

Commits
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1c2abab Improved the sentence
c8c093a Update security.rst

book/security.rst

@@ -20,8 +20,8 @@ perform a certain action.
 .. image:: /images/book/security_authentication_authorization.png
    :align: center
 
-Since the best way to learn is to see an example, start by securing your
-application with HTTP Basic authentication.
+Since the best way to learn is to see an example, just imagine that you want
+to secure your application with HTTP Basic authentication.
 
 .. note::