minor symfony#10323 Fix a typo in security/impersonating_user (nouwaarom)

This PR was merged into the 2.8 branch.

Discussion
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Fix a typo in security/impersonating_user

Commits
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cdb0e4f Fix a typo in security/impersonating_user.rst

Author: javiereguiluz

security/impersonating_user.rst

@@ -91,7 +91,7 @@ to show a link to exit impersonation:
 
 In some cases you may need to get the object that represents the impersonator
 user rather than the impersonated user. Use the following snippet to iterate
-over the user's roles until you find one that a ``SwitchUserRole`` object::
+over the user's roles until you find one that is a ``SwitchUserRole`` object::
 
     use Symfony\Component\Security\Core\Role\SwitchUserRole;